Comment 2 for bug 991695

It doesn't seem to be a ubuntu-tweak problem after all... whether you use it or background-settings to configure a desktop wallpaper, the problem will eventually occur.

There is however no consistency... on occasion one of the other std wallpapers may also lock up the login process as well. If one configures the greeter screen to have the same background as the current desktop which is an image from ~/Pictures, then the and the login process will also lock up. Using gradient colors instead of a wallpaper is one surefire workaround.