The big thing I'm thinking of are of course the original paintings. At least we know that MI2, FOA and Rebel Assault 2 are still in Purcell's and Eaken's possessions. As for stuff like MI1, MM, Zak or The Dig, they must be stored somewhere... I hope. Loom is an interesting one, because it's not a painting but a coloured pencil drawing. So most likely it was done on paper and is thus not as sturdy as paintings made on cardboard, MDF or canvas.
Interestingly, GameHistoryOrg's Frank Cifaldi did initially have the MI1 poster photographed at a specialized vendor, but the quality couldn't compete with a flatbed scan, so he ended up scanning it on his A3 scanner.
Edit: According to Frank, Steve Purcell confirmed that the poster's colors are accurate. So that settles that.