Also worth mentioning..
When you design your hands, you'll want to either design them at 640x640 as dubblebee says or make sure your image is centered however you want to use it.
In the editor, you can choose the anchor point for your hands. I think it defaults to center, so if you use a picture consider it will rotate around that anchor point. If you change the anchor point to the bottom or the top it will rotate around that part of the image.
Here's what I mean: