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yeah you can, the dimensions have to be 320x480 and it must be a PNG file named "Wallpaper"
then you SSH into your ipod and go to the Themes folder (assuming you installed Winterboard) and create a new directory. put your Wallpaper in there.
Then select the theme from winterboard.
 
or you can just use winterboard and customize to handle all of that. Those apps do all the dirty work for you. That is how I do it with mine.
 
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