Traction is only designed to have the global site background. You're saying you want a 2nd background in the inner content area, and you want it to change on a per-page basis?
If that's what you want, it's definitely something that would require custom coding, and is outside normal support. I suggest trying the Hire-a-Coder
area to see if you can find someone to do it.
If you find someone willing, I may be able to point them in the right direction, but it'll definitely require PHP changes in core framework files that are likely to be replaced in future patches.