IMHO this is why text links are much better, especially when they are keyword/content relevant!
Also, keep in mind that Google has differents robots.
I have noticed that once thier imagebot crawled my site and began listing pics in the GoogleImages:
My SERP increased
My
PR increased
My visitors increased
Google loves Google. Also, Google loves secrecy.
Just because G doesn't show a linkback listing, doesn't mean you're not getting the benefit having the banner or link. I think if it's in the Images side of Google, you'll still get residual benefits though there's no way to confirm or get Google to deny ....
Good luck and happy surfing ~