hold your finger on the picture (on the ipod) then click save, Go to your saved pictures and select for wall paper.
Or from your computer. Find the picture, right click save then add it to a folder and go to itunes. In Itunes go to your ipod and sync that folder. Then go to your pictures in the ipod and then make it your wallpaper.
The first meathod makes a bad quaility image. And if you mean background as in behind your apps on the home screen, then you can't unless you jailbrake it. Theyre are tutorials on how to jailbrake on the internet