You can use this:
It's in spanish but it's very easy to use.
Scroll down past the jobs and hats (once you choose what you want),
the "imagen/avatar" creates an avatar
Then you have "crear una firma" wich is the part you want.
"Fondo" is the background you want for the sig (note the they ask you
to NOT remube the credit of the signature)
"fuente" is the font and "nombre o texto" is where you write the mensage you want in your sig.
Once you're done, click "Crear mi firma!" and you have a new sig.