It is possible, but not automatically. You have to copy your desired picture into the picture folder of the device and then pick it as screensaver.
Of course you have to manually change it every time you start a new book in that way, but that's the only way I can think of right now.
If you're looking for some good wallpapers check out
http://www.nook-look.com/. They are designed for the Nook but fit the 650 very well.