I did two small updates today on the Fedora Mate setup (using the old Gnome supported release instead of the new terrible desktop environment).
First off, by default in Mate there are no Shutdown or Restart buttons. In order to be able to reboot, you need to wait for 60 seconds to logoff to the login screen and then reboot.
You could fix that by installing the ConsoleKit and activating the service as described here.
Second, since there is no UI to change the background visible on login and for unlocking (the annoying fireworks background), it's located in /usr/share/backgrounds/beefy-miracle/default/standard named 'beefy-miracle.png' (normally), though it's really linked from /usr/share/backgrounds/default.png (which links to the first one). So feel free to replace it and solve your problems.