Screensaver Hiccups - Huh?

I use a file of jpegs for my scrensaver and have them change every 5 minutes but I've noticed that some of my pictures appear as if someone hit a zoom button - for example, a pic of my daughter's two Jack Russel dogs originally show both dogs each sitting on different brick steps, one behind the other. Every so ofteen, the screensaver will show the original pic - both dogs in full view - but then the next time it will only show the dog on the first step and a portion of the dog behind it! This happens randomly to all my pics. I checked iPhoto, as someone suggested to see if there is a zoom setting or something that would cause this - nothing!

Any ideas? Not a show stopper but very puzzling.