Wallpaper is an image used as a background of a computer screen or mobile communications device. As a GeneRally fan you just have to use one :)
Is a piece of cake. One way to do it is the following:
You are going to need a suitable program for photos/picture editing. I'm using Photoshop but I suggest you try GIMP which is quite OK and free to use.
Everything else is left to your imagination and skills. Have fun and don't forget to share your creations :)
You also want to check this thread to see what kind of wallpapers other folks already created.