I've been wondering the same thing. I have been adding snippets to some profiles of old census transcriptions from public domain documents that have been scanned and are hosted by Google Books and I would like to provide a link back to the source.
Like you, I noticed that the comment section is not wikified, so URLs don't automatically become links. Using the biography of a related profile seems like a work-around to me. If the image is linked to multiple profiles, should I copy that info to all the profiles?
Perhaps adding a "caption" field to the image allowing freeform wiki text could be used to add source info or other relavant information.