Looks like there's a minor bug with this though. If pasting while viewing an activity stream it doesn't expand the message nor activate the media button, but still does the upload. This is now fixed in latest available CB Gallery build.
forge.joomlapolis.com/issues/7754
Now when looking at a stream and you attempt to paste an image it will expand the message display and activate the media button automatically so it no longer requires 2 manual clicks to reach that step. This is likely the behavior you were expecting.