Most of the previous posters are right. I have read up a bit into copyright law and it is lot more strict than many people think, mostly because everyone breaks it.
Basically, you cannot do it legally. If you do it, there is a large chance that you will get away with it 100%, unless your game becomes very popular.
If you ask permission, you probably will not get it, as Ashley has said.
It's your decision. It's likely that nobody is going to stop you if you choose the illegal route, but it is illegal, even if your game is free.
I would suggest that you make your own graphics, or use free sprite packs.