country flags overlaying slide show photos

I have my screen saver set to show a random slideshow from iphoto. Lately I have been getting low res images of flags from many countries showing up overlaid on the screen. I cannot find a virus using mcafee software. Anyone have a suggestion?

You know, when I had to rebuild my iPhoto database, some photos showed up that I had no recollection of seeing before. They were cute kids playing. I checked with other family members, who said they did not recognize them, either. Then later, I went to an Apple store, and on a demo they had running, I saw pictures of a hot air balloon that we own. I asked the clerk about it -- where did the pictures come from. He said that the computers ship with sample photos! So -- I am thinking the kids I found on mine were part of the sample that had been deleted and the rebuilding brought back. Maybe your flag photos are the same issue -- samples that are hidden on your hard drive?