The magic number turns out to be 381 pixels high! (It chops 19 pixels off the bottom. God knows why and probably so do the Atari ST developers.
Here's what I had to do. I've used another image after being driven mad by the Fifth element lol
I have however managed to make the thing which I'll include here. Works on Deskpic without being chopped now.
Here's a Fifth Element Desk Pic:
