The Google AdSense blog wrote a much needed Ad and image placement: a policy clarification. There has always been confusion about Google's policy about placing images near ads, to increase ones click-through rate on those ads. Here is a quote:
We ask that publishers not line up images and ads in a way that suggests a relationship between the images and the ads. If your visitors believe that the images and the ads are directly associated, or that the advertiser is offering the exact item found in the neighboring image, they may click the ad expecting to find something that isn't actually being offered. That's not a good experience for users or advertisers.
The Google blog has more details with examples of what does not comply with the terms of service.
Over the years, there has been a lot of confusion about this. Google recently was caught show casing a site with images near ads. And we have covered ways to use images to increase your CTR, plus tools for wordpress to make that easier.
Most AdSense advertisers are a bit upset by this clarification.
Forum discussion at DigitalPoint Forums (also here & here) and WebmasterWorld.
