the part where you take the painting is necessary to do, once she starts chasing you you have to just go left every turn, she wont be able to catch up to you, after the scene ends you can get the knife and then you can use it on the foot :
im sure all you have to do is to go thru the same room again to get the knife? when the scene ends youll need to go inside the painting lady room again and get the black object
OR youre talking about the KNIFE itself not the black object, to get the knife you have to interact with the black object and ask sai to cut himself to get the knife