There is much speculation about what is good and bad post-Panda. Almost all of it is speculation and guesses, including my suggestion that webmasters avoid using images that are the exact size of common ad formats.
If I was Google I would want to reward pages with high content & low advertisement ratio, and penalize the opposite. I am not saying Google does this. Just to be safe I would make sure your images are not the same size as common ad formats (
Common Google Ad Sizes [google.com]).
Regardless if Google is or is not doing this, using uncommon image sizes will also help to avoid issues with adblockers. If you have an affiliate ad on your site, adblockers could be hiding it from 15% of your users. So between Google paranoia and adblocker reality I think it would be smart to use uncommon image sizes.
