Well first off if your running a stock rom you can check this link out it's what I used for changing my wallpaper.
http://forum.ppcgeeks.com/showthread.php?t=94821
Here's one to change the overall look of you stock rom,
http://forum.ppcgeeks.com/showthread.php?t=91156
and you can search through this forum for more people who have changed their wallpapers and overall theme.
http://forum.ppcgeeks.com/forumdisplay.php?f=109
Don't foget to thank the creators if you like it,
Hope this helps to get you started.