I have now created a signature file by adding a signature
in the bottom right hand corner. You can use anything and
any letter type, just remember that some fonts are copyrighted
and that some logos are registered trademarks. But...
I am not going into the legalities here.
This is just a way of showing you HOW to do it.

Save the file somewhere where you will not accidentally delete it
as your action will retrieve it from this location every time.
I like to place mine somewhere in my Adobe folder...

And save it as a GIF, PSD or TIFF to preserve the layer's transparency.