Ah Grasshopper! That is a good question! Please to observe:

EDIT: Opps! I used a png so it was transparent.....here it's a jpg, so the alpha shows up black all around it.
The image of the Cathedral is 400 x 160. But it's on a 512 x 256 image. The boarder all around is transparent. This is needed to put linking panels in the GUIBook pop up.
If you look at the Myst V binks, all the movies that say: "WithAlpha" are in this same format, and is why it was presented on the book.
The movies without the alpha are for the books just laying on the stand! They didn't use animated textures. They actually used a .bik move (I think animated texture would be better and smaller file sizes).
I can squeeze the movie down to 400 x 160, but it needs to rest on a 512 x 256 transparent background. I could have sworn i had a util that would let me do that.....of course I'm drugged up on some pain killers right now due to a tooth ache........so I'm a bit woozy........