XP GP Editor / Desktop

I run two computer labs with over 50 workstations and I have set the GP so the desktop cannot be changed. (I specified the located Image which is now static / permanent) However, the desktop image only partially loads (from top to bottom, around 10% of the total resolution) and the rest is the default solid color of the desktop. I can hit F5 to fix this, but only manually when each workstation loads up. Does anyone have ideas on how to make the desktop refresh automatically?