This link will direct you to a YouTube page with Photoshop tutorials on them. I've included "signature" in the search because making forum signatures can be an art on itself. (Come to think of it, there's barely anyone who has unique/custom signatures on RFG...)
I think it'll be easiest to use Photoshop. (for a good result atleast) Making a signature is somewhat comparable to Cover Art so I'm sure these tutorials will help you on your way.
Tutorials.
EDIT: You will need to have a render from somewhere to use as a starting point for the cover art.