When I was setting up UpDox for a client, I learned a trick from them for handling that - you have to take the signature into an editing program and select and drop out of of the white so all you have left is a truly transparent signature.
It takes some time, and if you don't use image editing software regularly, I would make incremental saves as you go, because you'll make mistakes.
Once done, you have a signature that you can use in AC, and it works great in other uses because any lines you drag it on top of show through like a regular signature.
