There a lot more work involved ina wallpaper and since is much bigger it will require moe time to make. You have to make it detailed and make sure not to leave out spots, then it will look plain. It takes more time, but its better than making sigs IMO.
Basically it is the same as making a sig, other than the size and time... well, just like Kurrupt said, you can't mis any spots or it messes the whole thing up.