Creating a signature in Preview on Snow Leopard?

I have Mac OS X 10.6 and a fillable PDF form that requires a signature. I used an image editing program to create an image file with a signature I created by just dragging my mouse to mimic my signature.

In the past I have used this trick successfully, I think. For some reason, I can't past into this PDF document and the Preview.app annotation tools don't have a pen tool. Any thoughts? Thanks!