Call of Duty puts players in the shoes of a foot soldier in the American, British, and Russian armies during the Second World War. These soldiers aren't the one-man armies of other games; these soldiers are just another cog in the war machine.
This mod changes the multi-player background in Call of Duty and Call Of Duty United Offensive multi-player. The game will load a single image instead of two images and mod name, version number and so forth can be added to the menu background.
I know this is very old, but if anyone sees this, how do I actually add my background to this, sorry I'm new to this
You download and place zzzz_backgroundmod.pk3 into the Main folder. You can open pk3 files with for example WinRAR, and you can see a strict folder structure. Inside the archive find ui_mp\assets\main_background.dds. That's the file you want to edit. Dds files can be opened for example in Photoshop with the required plugins which you can easily find on the web. Pay attention that this background is 1024x1024 sized, meanwhile you display is most certainly not. If you want to make it look undistorted you can scale down an 1920x1080 picture to 1024x1024 to stretch well.