Whoa, I never realised that. So, because your site is loaded in an iframe, the image-searcher's visit doesn't count as an impression (hence you won't get paid for pay-per-impression ads)!? And worse, if someone clicks on one of your AdSense ads while it's still within the iframe, the click won't count as originating from your site, and so you won't get paid for that either!?
When I'm Google-image-searching, I get pretty annoyed with sites that break me out of the frame, but now I can better understand why they do it!