Does it matter whether they are relevant or not, and is it alright to make them random or do they have to be static?
i dont think its easy to detect the ad and display a relevent pic; also question is not quite clear ; if you are talking to use image next to ad to increase CTR then how can you detect the nature of ad? and if you say it would depend on my content then surely that pic should be related to your content!
Like if I have a pic of Halo 3 and the ad is for Halo 3, is that legal? And if I have a different pic every time you refresh, is that legal?
Fact is, Google insists you can't have a picture that misleads people into thinking your picture is part of the ad (which is what putting relevant pictures next to the ads is all about), but I don't see them enforcing it at all. Take a look at this site: http://www.fastcoolcars.com/ He's not only deceiving the visitors with car pictures next to car ads, but he has no borders whatsoever. And this guy's still in business. Adsense just doesn't seem to care, or they are selective about their enforcement. So they say it's not allowed, but would theY REALLY do something about it? Your guess is as good as mine.
I've created a few informational sites using the pictures-next-to-square-ad-unit trick, and Google actually gave their blessings on the placement of it. There's nothing to worry about as long as you keep the images suitably generic and don't do anything silly like using arrows or flashing lights.