The default login screen background for Linux Mint 11 is less than pretty. Cold would be a better word. Fortunately its easy enough to change.
$ sudo bash # cd /usr/share/backgrounds/linuxmint # mv default_background.jpg default_background.jpg.orig # ln -s /full/path/to/new/background/image_file.jpg ./default_background.jpg # exit
The image should now be used the next time you see the login screen. If this doesn’t wind up looking the right way the link we just created can be removed and the original file renamed back into place.