Transparancy. When I set a image as my BG and am not just using a theme it ask me to set the transparancy lvl of that image before the background is applied. If you don't get this option I sugest you just download and use a theme.
this is a good place to start.
http://forum.ppcgeeks.com/showthread.php?t=8211
Mega also has a very nice site that provides themes by request. But hook him up and at least click on an add or something.
http://www.ppcthemes.net