Game.registerMod("<MOD ID GOES HERE>", {

init: function() {

Game.Loader.Replace('bgBlue.jpg',this.dir + '/<newwallpepergoeshere>.jpg');

}
});

You can use this code. To make this process easier you can install any mod to use it as a framework for yours. Simply paste this code up above into the main.js file with Notepad++ or your program of choice, give your mod an ID for the game to recognize, it can be anything, and then put the image you want to replace the wallpaper with into the mod folder and make sure it's a 512x512 image, otherwise the game will likely stretch the texture to fit the dimensions, making it look ugly. Make sure the second filename in the code matches the one you want to be replacing the original wallpaper. You can also change "bgBlue to something like "bgMint" or "bgRed", the names for each wallpaper will be in the 'img' folder located in the game's files.
You'll also want to edit the info.txt file to match whatever mod ID you chose to put into the main.js file, otherwise the game won't recognize the mod.

Theoretically you could also simply change the game's actual texture file but doing this isn't the best idea since if the game updates it might reset the image.

Hope this helps! If you need help feel free to shoot me a friend request, my knowledge of making mods only extends as far as texture changes and audio file replacements, all I've learned is just from copying code from other workshop mods and trying to figure out what the code does from there.