Google has approved using background images on AdSense…

We have contacted Google multiple times and they have continually said that if the backgrounds don’t bring undue attention to your ads, then you are OK. Of course, if you have any concerns, or if you are unsure of the implementation you have, please contact Google directly.

Simply and to the point, Google insists that background images are fine so long as the ads are clearly portrayed as ads, and clearly distinguished from the content.

Here is a quote from the Google reply email (only an excerpt shown because we also asked to get our email address changed, and no one needs to read about that…)

…
I have also reviewed the content and ad implementation on your pages and it currently complies with our program policies. We look forward to your continued success and participation in the AdSense program.

Sincerely,

XXXXX
The Google AdSense Team

UPDATE: Here is also an emailed response that Appliblog got from Google.