I have a Samsung S2 and until recently changing the lock screen was fine, however suddenly (potentially after apps were synced), I am unable to change my lock screen using a gallery photo, using both the change lockscreen prompt through Gallery itself, from holding down the homescreen as well as through the Settings. I am only able to change it to one of the factory Wallpapers.
Changing the home screen is fine, and as far as I know my only third-party editing app is TwCrop, which I got to try and fix the problem (it didn't).