Several times I have added a new watch face, and then deleted it a short time later because I didn't like it. It's not uncommon, when I delete a face, for other faces to start losing image files.
I started checking, and sure enough, almost every time I load a new face, images from previously loaded faces will get overwritten by images from the new face.
I have about 40 loaded. I don;t know if that is too many, but I basically have to wipe out the BeautifulWatches folder every time it happens and re-import all of my faces.