I have some ads on my pages which are an unusual format. They also don't have "Ads by Google" on them. I have only seen Adsense ads on my pages that have "Ads by Google" on them. The ad occupies the top half of the available space with blank space below.

The ad is for that well known online auction site. Any ideas if this is a valid ad or is something else going on? I have looked on my PC and it all looks OK, as does the source when I view it from the page online.

Any advice will be appreciated.