I think just Last Crusade and Loom are missing now. I don't think the Last Crusade box art will make a good poster, but the Loom one might come out well.
In addition, although these aren't LucasArts games, they are developed by the same people:
Telltale games released some highish res images of box art for Season One and Season Two, renamed Sam & Max Save the World and Sam & Max Beyond Time and Space. They may make half-decent posters.