As a general matter, under US law (and the extensive precedents set by Google), text snippets and image thumbnails of the sort that could be used in a twitter card are likely to be fair use.
That said, we should still try to display as much licensing information as we practically can, and consider what to do in cases where we can't. (e.g., if we can't parse the license information of an image, should we rely on fair use or should we just not show the image?)
Opening this bug to track those sorts of questions. Is there any way (beta labs or some such) to see what the extension currently does about licensing?