If I'm understanding the question correctly, you want to have the Eye logo in posts made by a bot in [something that's just like] Slack.

Looks like http://your-phabricator.host/rsrc/image/logo/light-eye.png works to deliver this file in most cases, but this is a more-or-less a hack that might stop working some day. Otherwise, you can just copy the file somewhere, or use the favicon.