How do I upload more than one image to use as a signature? I provide support to three managers and usually need to add their signatures to documents.

I provide support to three managers and usually need to add their signatures to PDF documents.  Somehow two of the signatures are displaying in the signature drop down (not sure how those got there as I am a new user to this - perhaps the user before had them loaded), and I would like to add another signature to the drop down, however I don't see an option to add one.