I think they ought to give a few options:
1. The cover what you are reading (or perhaps what you were last reading since you could be reading multiple items)
2. A specific image rather than the random image (either user uploaded or from what was already provided). Afterall, I actually find some of the images cool.
-- As a subnote, I think it would be nice if you could remove images you don't like.
3. Any image a user provides (randomly or not).
My wife and I both received Kindle 3, and without covers, yet, it is difficult for us to know which one is which since the screensaver images are random from the same set. Right now our only solution is to turn one Kindle completely off so there is no image on one. Of course, once we have a cover on each (different colors), this issue will be gone.