I just got my macbook 8 days ago. Already love it. But about 3 days ago I changed the background to black, and now it won't change to anything else. The picture in the system preferences/desktop & screensaver box changes, but the background won't. I run OS X Lion 10.7 (not sure if I wrote it out right or not, but that is the gist). I haven't updated it ever. Any ideas?