outrun - 7:25 am on Feb 22, 2009 (gmt 0)

Im not sure if things have changed but AdsenseAdvisor has said

[webmasterworld.com...] this is a familiar topic. Honestly, our policy specialists and I aren't able to offer you a blanket "yes" or "no" on placing images behind ads because we've seen so many variations of it. In many cases it depends on the specific site and context in which the ad and image are placed. In general, we don't encourage the use of programs like this because there's a good chance that placing images behind your ads will bring unnatural attention to the ads. So to be on the safe side, I suggest you avoid this type of implementation.