My professor just posted the answer, in case anybody was interested!
The "mistake" in the picture of Pandora and its parent, giant planet is this: the giant planet is rendered in deep blue colors, like the colors of Uranus and Neptune in our Solar System. The parent planet clearly is a giant...
My professor showed us this photo from Avatar and asked if anybody could find a mistake within it, but I can't see it! Can you tell what is wrong with it?
http://img291.imageshack.us/img291/6331/pandoraavatar.jpg
The mass closest to us is Pandora and there is a small thing off to the right...