This is the stack trace which i am getting in my logs

[17-Apr-2017 13:02:26 UTC] [2017-04-17 13:02:26] EXCEPTION: (Error) Call to undefined function imagecreatefromstring() at [<phabricator>/src/applications/files/builtin/PhabricatorFilesComposeAvatarBuiltinFile.php:66]
[17-Apr-2017 13:02:26 UTC] arcanist(head=master, ref.master=a59cfca5f190), phabricator(head=master, ref.master=a56f9a1a5599), phutil(head=master, ref.master=6fe33623cda6)
[17-Apr-2017 13:02:26 UTC] #0 PhabricatorFilesComposeAvatarBuiltinFile::composeImage(string, string, array) called at [<phabricator>/src/applications/files/builtin/PhabricatorFilesComposeAvatarBuiltinFile.php:57]
[17-Apr-2017 13:02:26 UTC] #1 PhabricatorFilesComposeAvatarBuiltinFile::loadBuiltinFileData() called at [<phabricator>/src/applications/files/builtin/PhabricatorFilesComposeAvatarBuiltinFile.php:168]
[17-Apr-2017 13:02:26 UTC] #2 PhabricatorFilesComposeAvatarBuiltinFile::getUserProfileImageFile(string) called at [<phabricator>/src/applications/people/cache/PhabricatorUserProfileImageCacheType.php:54]
[17-Apr-2017 13:02:26 UTC] #3 PhabricatorUserProfileImageCacheType::newValueForUsers(string, array) called at [<phabricator>/src/applications/people/storage/PhabricatorUser.php:1563]
[17-Apr-2017 13:02:26 UTC] #4 PhabricatorUser::requireCacheData(string) called at [<phabricator>/src/applications/people/storage/PhabricatorUser.php:838]
[17-Apr-2017 13:02:26 UTC] #5 PhabricatorUser::getProfileImageURI() called at [<phabricator>/src/applications/people/engineextension/PeopleMainMenuBarExtension.php:56]
[17-Apr-2017 13:02:26 UTC] #6 PeopleMainMenuBarExtension::newDropdown(PhabricatorUser, PhabricatorSettingsApplication) called at [<phabricator>/src/applications/people/engineextension/PeopleMainMenuBarExtension.php:19]
[17-Apr-2017 13:02:26 UTC] #7 PeopleMainMenuBarExtension::buildMainMenus() called at [<phabricator>/src/view/page/menu/PhabricatorMainMenuView.php:107]
[17-Apr-2017 13:02:26 UTC] #8 PhabricatorMainMenuView::render() called at [<phabricator>/src/view/page/PhabricatorStandardPageView.php:380]
[17-Apr-2017 13:02:26 UTC] #9 PhabricatorStandardPageView::willRenderPage() called at [<phabricator>/src/view/page/AphrontPageView.php:46]
[17-Apr-2017 13:02:26 UTC] #10 AphrontPageView::render() called at [<phabricator>/src/view/page/PhabricatorStandardPageView.php:883]
[17-Apr-2017 13:02:26 UTC] #11 PhabricatorStandardPageView::produceAphrontResponse() called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:649]
[17-Apr-2017 13:02:26 UTC] #12 AphrontApplicationConfiguration::produceResponse(AphrontRequest, PhabricatorStandardPageView) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:278]
[17-Apr-2017 13:02:26 UTC] #13 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:177]
[17-Apr-2017 13:02:26 UTC] #14 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:17]