We reported earlier this month that Yahoo! has taken a stronger stance against images near your ads. Whereas Google allows it (also see here & here), Yahoo! wants to set themselves apart from Google and not allow it. The thing is, most Yahoo! Publisher Network folks are accustomed to Google's rules, so Yahoo! has to educate.
A DigitalPoint Forums thread shows that Yahoo! is sending out emails to publishers who are using the image near ads method to increase their click-through rates. The emails read like this;
You are in violation of our obscuring guideline. A user must be able to tell the difference between the Ad Unit and other content on your page. The Ad Unit should be separated from other page content, including images, by a border, a clear difference in color, or some other means. Images may not be used to draw attention to the Ad Unit or as a click incentive. I hope this information helps you to understand the requirements more clearly. Please let us know if you have further questions.
But many are written directly in relation to the site and give specific examples to help each publisher.
Forum discussion at DigitalPoint Forums.