To upload your signature it's fairly similar to how you upload your avatar. If you go to your profile, and then click "Profile" you should be able to get to a page with a lot of different options, if you click "Edit Signature" you should be able to get the rest from there.
You can either copy a picture link to get your pic as your signature, or you could just upload it by taking the picture directly from your computer.
Hope this was helpful to you!