I think you'll find this is just the scaling feature rather than anything to do with zooming. Way back when, the photo screen saver couldn't handle pics greater than 1024 pixels (I think, can't remember if this is vertical, horizontal or both!).
When you put pics in there that are larger than this, it would start cycling back and forth between 2 specific pics, for ages, before moving on, or even between a black and white screen. This was fixed up simply in the latest release by adding a scaling function (you can see the Device Data for this in the device) that prescales each image down if it is larger than this, first before displaying it. I think that is what you are seeing. If you remove all images but a handful of small ones, you should see them all appear unscaled and at their correct size (ie with blank borders around them).
Also, tail the PSS log file, you can see the messages being logged in there about pictures being scaled so that you can confirm what is happening.