What Cathbert should work no problems, might need a little tweaking but that's about it. Only thing is this wouldnt fix your "left bg image" issue.
If by left bg image you mean the little blue bars on the sides of your layout, you could probably do it by having five columns instead of three.
Something along the lines of :
Code:
#leftside
#leftside-bg (width will be the one of your left bg image - set up background for that column and you're good.)
#center
#rightside-bg (same as #leftside-bg, even if you don't use a bg image for the right side, keeps everything centered)
#rightside
Something similar to this should work. You should have everything centered and it should also fix your issue with the left bg image.
Didnt test this out, but you get the general idea.