i know you can set a picture for the homescreen backround, but can you have a picture in the back on the other screen (where all the other icons are)??

Background is background. There is no function to have different backgrounds for the two screens.

I'm using a theme that uses your home screen picture on all the screens even the folders. The name on my 8900 is stock_calendar_8900. I found it at blackberrypark.com/2009/stock-today-plus-for-the-blackberry-curve-8900/
You can give it a try its a free theme.