Currently Being ModeratedJul 31, 2012 4:47 AM (in response to clank31)
I had this same issue yesterday when attempting to change desktop image in system preferences. After trying several potential solutions including reloading Mountain Lion I sought the help of Applecare. Apparently there is a glitch affecting the interraction of iPhoto and System Preferences Desktop and Screensaver. When I was put in contact with Level 2 support Applecare was able to fix the issue. They said this issue is rare but of course if it happens to you that doesn't matter. To be on the safe side I suggest not trying to transfer images from iPhoto into Preferences Desktop and Screensaver unless absolutely necessary as doing so seemed to trigger the freeze problem. if the freeze initiates until resolved you will be unable to change desktop / screensaver selection and also you will need to force quit system preferences in the Apple menu to get it off your screen. Thank God for Applecare!