#mainWrapper is the entire area of the page not counting blank/background space around it, if any.
#contentMainWrapper is the area between the header and the footer.
Assuming your page will be a fixed width and your header and footer will be fixed heights, cut your Photoshop image into sections that fit the header and footer, and apply them as background-images to #headerWrapper and #mainWrapper respectively. (The footer area is made up of two or three separate elements, and using the bottom of #mainWrapper may be easiest.)
The remaining center part of the design will be the background-image for
#contentMainWrapper. This part will have to be able to repeat vertically to fit any height of page content.
Code:
#headerWrapper {background-image: url(../images/topbg.gif); background-position: top center; background-repeat: no-repeat;}
#mainWrapper {background-image: url(../images/bottombg.gif); background-position: bottom center; background-repeat: no-repeat;}
#contentMainWrapper {background-image: url(../images/middlebg.gif); background-position: top center; background-repeat: repeat-y;}
There are other possible ways of organizing this as well, and some may be better for certain designs.