You can extract images, sound and other things with SCUMM Revisited (
http://scummrev.mixnmojo.com) and then you can edit them. I'm sorry but I don't know any way of re-inserting resources into LucasArt's data files to show the modified things into the game.
As I explained to others in this forum (
http://66.250.145.38/showthread.php?s=&threadid=68289), in-game texts are editable, but you can't put longer strings than original ones.