Multi Monitor Wallpaper

i use a triple-head configuration with my center monitor specified as primary display. i have made some 3x Wide screen wallpapers, and i set them as the desktop backgrounds. it works fine with one caveat:

the images left most border starts on the center display --the image is triple wide, but the right-most third wraps around the the left display.

any ideas on how to properly center the image? i need the center display to be the primary as certain applications throw a fit when they are run on a non-primary channel.