Kaiser Four comes with about 100 wallpapers for the game screen.
Here is a link to alot more that you can download and use. You dont have to use the ones that come with the game. Just download them as 1366x768 resolutions and they will fit perfect.
Try the Textures or the Vectors and be sure to use darker images because as you will see the text colors are lighter and the background colors and wallpapers should be dark.
http://wallpaperscraft.com (http://wallpaperscraft.com)