what does the creature do with william's portrait?

The creature makes it look like Justine has taken it. This is the reason she's tried for muurder.

He places it int Justine's pocket when he finds her sleeping in the barn, thereby planting "evidence" that she killed William. He had learned mischief from the books he read, and he was mad at humankind, so he decided to do some mischief of his own.