They would own the picture issue so that you wouldn't have much to stand on in regards to likeness of the Shrek character. The other issue, well, that'd be hard. IIRC, in the movie, the character was referred to as Shrek. I don't recall "Shrekkie" being used so that, I would think, would be to your advantage. You might want to visit an actual lawyer about it and find out what kind of "legs" you stand on.