To be honest, nobody cares if you have ripped graphics in your game. LucasArts don't tend to care about this kind of stuff, they haven't intervened in fan games since about 2002 or so. I think they put a C&D on
The Devil's Triangle (a MI fangame) though I remember playing that, so maybe not. Fate of Monkey Island 2*. But if you consider the wealth of fan games out there using LEC graphics, they're highly unlikely to waste the time doing anything about you. Of course you ideally don't get noticed until the game is released. On the whole, I wouldn't worry about it.
*(Edit: Changed the name of the game LEC stopped.)