screen saver bug

Hi folks.

Once my MacPro (OS 10.8.2) goes to "screen saver", upon my return to my Mac, the image on screen is basically a snap-shot of my desktop, yet it's not my actual desktop... As I begin to move my mouse around, a "square" (25 pixels by 25 pixels??) which is now my cursor "erases" this stale desktop image, to reveal the gray background of the sign-in screen.

After sufficiently "erasing" the stale desktop image picture to reveal the login account I wish to use, I can now click on that account, enter my password, and a normal login sequence follows.

This has GOT to be a bug of some kind, never seen this one before, but it happens everytime, regardless of which screen saver setting I use.

Supplimental info: Harddrives aren't set to sleep, neither is the computer, just the display. My desktop image is a picture I've taken, not one of the stock Mac OS desktop images. I'm viewing through an older Cinema display, the resolution of which I can no longer set due to Mtn Lion's auto pverride (anti)-feature.

Any suggestions form anyone? Anyone else experiencing such silliness? Thanks in advance for anyone's assistance!