I got the same message today. The specific ad block they referenced, which has not had any changes in more than 6 months, specifies text only. I believe the rejected ad was someone who wanted to post an image ad in that space. I reject image ads because my site is a fairly narrow special interest niche, and in previous tests, all the image ads I have ever seen on my site have poor to terrible contextual relevance - dieting and other unrelated topics. So I have no qualms about rejecting image ads - for me they result in reduced revenues in spite of strong suggestions otherwise by G-adsense staff. I know the specifics of my own site better than they do. I have not seen any reduced revenues in recent days or weeks.