I am a little confused as to what exactly you mean by "signature"?

Seems to be multiple options ranging from a simple text string of the actual user name who is creating the email through to a more complex "email signature block" of multiple lines of string text and eventually to an image that contains a "real signature" and contact details, again for the user actually creating the email.

Each of these will need a different solution, at least that is what it looks like to me.

AND... as I am sure you have seen from other posts, it always helps you get an answer if you attach a sample workbook to you request.
Cheers