Local MCX & Desktop images

Ok, so I have a setup with Local MCX that places a desktop image using the plists. Here's the oddity. Every Friday the desktop image changes in our environment (yeah, I know). So I wrote a script that downloads the new image and replaces the old one - it has the same name so it should just update, right? No. The source image is updated, but the image on the user desktop is not updating. I know it's cached somewhere, and I've been looking but does anyone know where the cached image is, so I can delete it in my script, and it will create a new cached image based on the default? Thanks...