I'm with Skully on this. It's easy to cross your eyes the right way, but actually figuring out what the image IS is incredibly difficult. After lego mentioned what it was, I was able to go back and see the details, but when I first looked at it, it just looked like random blobs.
I think the reason is because I'm using the crossing-your-eyes technique, so instead of the picture popping out at me, it caves in, IE the background is in front.
If I was any good at the parallel eyes technique I would try that and compare the two.
EDIT: just out of curiosity, what would happen if you inverted the colors used in the grayscale image? Do you think that would swap the foreground and background positions? It's worth a try.