This will show you how to change any picture on your iPod, including your background. Unfortunately this does not work on Mac computers, and like many other iPod hacks, it doesn't work on the Shuffle, 2nd and 3rd generation Nano, and the new Classic or iPod touch due to higher levels of encryption.
This hack will void your iPod warranty from Apple, although if you just rewrite your original firmware or restore your iPod there is no way for anyone to tell you did anything but use it like anyone else :-)
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First you need to enable disk use on your iPod. This can be done by :
Hooking-up your iPod to your PC
Opening up iTunes
Select your iPod and click the checkbox that says Enable Disk use. (if it isn't already checked)