Desktop/Screensaver Problem

The other day I went to change my desktop background and I noticed that the Desktop/Screensaver button has disappeared. So, I googled my problem and found this: http://forums.macrumors.com/showthread.php?t=327807. I tried what was suggested there and it didn't help anything. So, I left it alone and today I was adding a lot of new songs to my itunes and it was taking a while so I just left my computer alone. When I looked at it again, a screensaver had come on and I did not have a screensaver on my computer turned on. So, I went to move the mouse and my computer had frozen. I then restarted my computer and the picture I had for my background was gone and the one of the galaxy was there instead. I need help how to fix this. I don't know what I did or what to do.

 

no there is no button there anymore. i can't change my picture.