by Xanayoshi » Mon Apr 07, 2014 1:17 pm
You need to change the background for lightdm. Lightdm's background can be affected by Nautilus. Mate is built by Mint and generally uses mdm. Mate uses caja.
Hmmm
I am not on a Linux PC unfortunately and even more so not on an Ultimate Edition. However, I would as a quick fix, open a root file manager and navigate to usr/share/backgrounds. Locate the background that you see on the login. If you are lucky it will say default or something something ubuntu. If you see this, find a wall that you would prefer and drop it in this folder, renaming it the same as the background file. This should switch it out. Backwards, and I am sure there is an easier way, but it should do the trick.