Here's what you are looking for.
http://www.gpain.com/bbs4/zboard.php?id=wi...c=asc&no=39
(Sorry, the whole page is in Korean)
1) Rename the image file as body.png
2) Put bg.gpu and body.png in the root directory of SD card.
3) Run bg.gpu.
*Make sure the size of bacground image is 320X240.