Desktop Display - Reducing

Hi all, I am brand spankers to this forum, so please pardon my ignorance if this is an old question. I have a number of pictures from my iphoto album, as my desktop, but some of them are getting smaller each day! I mean to say, that I have the preferecens set to show a new picture every minute (goldfish mentality, but hey!) and random. As they appear, some, not all, are smaller than they were.

Any ideas?