Yeah, but I think the upload instructions still say that it has to be a .bmp.
(15 seconds later)
Ok, I just checked. The field descriptors are like this:
*Preview Image (.jpg only):
*Bitmap Zip:
which implies that the wall should be a bitmap instead of a .jpg or other format. Before I knew better, I converted a couple of walls from .jpg to .bmp in the past when uploading because of this.