You can drag and drop images into remarkup areas, but only from the desktop (and from some browsers, the clipboard). We should either support also dragging in from other web browsers, or not pretend that we support it.
I just found the following in my logfile, after trying to upload a Conpherence image from within the website itself (i.e. dragging and dropping an image from my Phabricator instance website into the upload field of Conpherence, without saving it locally)
PHP message: [2015-05-05 19:11:16] ERROR 2: imagecreatefromstring(): Data is not in a recognized format at [/opt/phabricator/phabricator/src/applications/files/PhabricatorImageTransformer.php:198] PHP message: arcanist(head=master, ref.master=8919a9c5b55e), libphremoteuser(head=master, ref.master=1def4e2d7f07), phabricator(head=master, ref.master=92d399d086c9), phutil(head=master, ref.master=cb8cdccb1a50), sprint(head=master, ref.master=53f16c2f6fa0) PHP message: #0 imagecreatefromstring(string) called at [<phabricator>/src/applications/files/PhabricatorImageTransformer.php:198] PHP message: #1 PhabricatorImageTransformer::getPreviewDimensions(PhabricatorFile, integer) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:112] PHP message: #2 PhabricatorEmbedFileRemarkupRule::renderImageFile(PhabricatorFile, PhabricatorObjectHandle, array) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:47] PHP message: #3 PhabricatorEmbedFileRemarkupRule::renderObjectEmbed(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:114] PHP message: #4 PhabricatorObjectRemarkupRule::renderObjectEmbedForAnyMedia(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:368] PHP message: #5 PhabricatorObjectRemarkupRule::didMarkupText() called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:292] PHP message: #6 PhutilRemarkupEngine::postprocessText(array) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:138] PHP message: #7 PhabricatorMarkupEngine::process() called at [<phabricator>/src/applications/conpherence/ConpherenceTransactionRenderer.php:59] PHP message: #8 ConpherenceTransactionRenderer::renderTransactions(PhabricatorUser, ConpherenceThread, boolean, string) called at [<phabricator>/src/applications/conpherence/controller/ConpherenceViewController.php:70] PHP message: #9 ConpherenceViewController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:218] PHP message: #10 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:132] PHP message: #11 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:19] PHP message: [2015-05-05 19:11:16] ERROR 2: imagesx() expects parameter 1 to be resource, boolean given at [/opt/phabricator/phabricator/src/applications/files/PhabricatorImageTransformer.php:200] PHP message: arcanist(head=master, ref.master=8919a9c5b55e), libphremoteuser(head=master, ref.master=1def4e2d7f07), phabricator(head=master, ref.master=92d399d086c9), phutil(head=master, ref.master=cb8cdccb1a50), sprint(head=master, ref.master=53f16c2f6fa0) PHP message: #0 imagesx(boolean) called at [<phabricator>/src/applications/files/PhabricatorImageTransformer.php:200] PHP message: #1 PhabricatorImageTransformer::getPreviewDimensions(PhabricatorFile, integer) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:112] PHP message: #2 PhabricatorEmbedFileRemarkupRule::renderImageFile(PhabricatorFile, PhabricatorObjectHandle, array) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:47] PHP message: #3 PhabricatorEmbedFileRemarkupRule::renderObjectEmbed(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:114] PHP message: #4 PhabricatorObjectRemarkupRule::renderObjectEmbedForAnyMedia(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:368] PHP message: #5 PhabricatorObjectRemarkupRule::didMarkupText() called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:292] PHP message: #6 PhutilRemarkupEngine::postprocessText(array) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:138] PHP message: #7 PhabricatorMarkupEngine::process() called at [<phabricator>/src/applications/conpherence/ConpherenceTransactionRenderer.php:59] PHP message: #8 ConpherenceTransactionRenderer::renderTransactions(PhabricatorUser, ConpherenceThread, boolean, string) called at [<phabricator>/src/applications/conpherence/controller/ConpherenceViewController.php:70] PHP message: #9 ConpherenceViewController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:218] PHP message: #10 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:132] PHP message: #11 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:19] PHP message: [2015-05-05 19:11:16] ERROR 2: imagesy() expects parameter 1 to be resource, boolean given at [/opt/phabricator/phabricator/src/applications/files/PhabricatorImageTransformer.php:201] PHP message: arcanist(head=master, ref.master=8919a9c5b55e), libphremoteuser(head=master, ref.master=1def4e2d7f07), phabricator(head=master, ref.master=92d399d086c9), phutil(head=master, ref.master=cb8cdccb1a50), sprint(head=master, ref.master=53f16c2f6fa0) PHP message: #0 imagesy(boolean) called at [<phabricator>/src/applications/files/PhabricatorImageTransformer.php:201] PHP message: #1 PhabricatorImageTransformer::getPreviewDimensions(PhabricatorFile, integer) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:112] PHP message: #2 PhabricatorEmbedFileRemarkupRule::renderImageFile(PhabricatorFile, PhabricatorObjectHandle, array) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:47] PHP message: #3 PhabricatorEmbedFileRemarkupRule::renderObjectEmbed(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:114] PHP message: #4 PhabricatorObjectRemarkupRule::renderObjectEmbedForAnyMedia(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:368] PHP message: #5 PhabricatorObjectRemarkupRule::didMarkupText() called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:292] PHP message: #6 PhutilRemarkupEngine::postprocessText(array) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:138] PHP message: #7 PhabricatorMarkupEngine::process() called at [<phabricator>/src/applications/conpherence/ConpherenceTransactionRenderer.php:59] PHP message: #8 ConpherenceTransactionRenderer::renderTransactions(PhabricatorUser, ConpherenceThread, boolean, string) called at [<phabricator>/src/applications/conpherence/controller/ConpherenceViewController.php:70] PHP message: #9 ConpherenceViewController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:218] PHP message: #10 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:132] PHP message: #11 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:19] PHP message: [2015-05-05 19:11:16] ERROR 2: Division by zero at [/opt/phabricator/phabricator/src/applications/files/PhabricatorImageTransformer.php:204] PHP message: arcanist(head=master, ref.master=8919a9c5b55e), libphremot [Tue May 05 19:11:16.662984 2015] [proxy_fcgi:error] [pid 26102] [client <…>] AH01071: Got error 'd=master, ref.master=53f16c2f6fa0) PHP message: #0 PhabricatorImageTransformer::getPreviewDimensions(PhabricatorFile, integer) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:112] PHP message: #1 PhabricatorEmbedFileRemarkupRule::renderImageFile(PhabricatorFile, PhabricatorObjectHandle, array) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:47] PHP message: #2 PhabricatorEmbedFileRemarkupRule::renderObjectEmbed(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:114] PHP message: #3 PhabricatorObjectRemarkupRule::renderObjectEmbedForAnyMedia(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:368] PHP message: #4 PhabricatorObjectRemarkupRule::didMarkupText() called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:292] PHP message: #5 PhutilRemarkupEngine::postprocessText(array) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:138] PHP message: #6 PhabricatorMarkupEngine::process() called at [<phabricator>/src/applications/conpherence/ConpherenceTransactionRenderer.php:59] PHP message: #7 ConpherenceTransactionRenderer::renderTransactions(PhabricatorUser, ConpherenceThread, boolean, string) called at [<phabricator>/src/applications/conpherence/controller/ConpherenceViewController.php:70] PHP message: #8 ConpherenceViewController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:218] PHP message: #9 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:132] PHP message: #10 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:19] PHP message: [2015-05-05 19:11:16] ERROR 2: Division by zero at [/opt/phabricator/phabricator/src/applications/files/PhabricatorImageTransformer.php:204] PHP message: arcanist(head=master, ref.master=8919a9c5b55e), libphremoteuser(head=master, ref.master=1def4e2d7f07), phabricator(head=master, ref.master=92d399d086c9), phutil(head=master, ref.master=cb8cdccb1a50), sprint(head=master, ref.master=53f16c2f6fa0) PHP message: #0 PhabricatorImageTransformer::getPreviewDimensions(PhabricatorFile, integer) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:112] PHP message: #1 PhabricatorEmbedFileRemarkupRule::renderImageFile(PhabricatorFile, PhabricatorObjectHandle, array) called at [<phabricator>/src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php:47] PHP message: #2 PhabricatorEmbedFileRemarkupRule::renderObjectEmbed(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:114] PHP message: #3 PhabricatorObjectRemarkupRule::renderObjectEmbedForAnyMedia(PhabricatorFile, PhabricatorObjectHandle, string) called at [<phabricator>/src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php:368] PHP message: #4 PhabricatorObjectRemarkupRule::didMarkupText() called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:292] PHP message: #5 PhutilRemarkupEngine::postprocessText(array) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:138] PHP message: #6 PhabricatorMarkupEngine::process() called at [<phabricator>/src/applications/conpherence/ConpherenceTransactionRenderer.php:59] PHP message: #7 ConpherenceTransactionRenderer::renderTransactions(PhabricatorUser, ConpherenceThread, boolean, string) called at [<phabricator>/src/applications/conpherence/controller/ConpherenceViewController.php:70] PHP message: #8 ConpherenceViewController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:218] PHP message: #9 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:132] PHP message: #10 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:19]
On the site it did nothing, but apparently it created this error in the logfiles. Later I downloaded the image file and then uploaded it into the drag&drop area from a file manager, which did not create such an error and successfully uploaded the file.