Design Theme with internal picture

Hello all,
After searching the internet without finding any simple solution to my quest – I would really appreciate your help here…

I’m using Twenty Ten latest Version.

All I want to do is design the internal background of my theme; simply change the #fffff white colour into impressing picture.

I need to know how to add a background image into the white empty spaces in my theme such as: inside posts and pages, behind widgets and ect.

Do you have a simple solution for my need?
Also, how come I didn’t find any Plugin for it?

Thanks a lot,

Moria.