(Gmail) Sending emails from spreadsheet. How to add signature with image?

(Gmail) Sending emails from spreadsheet. How to add signature with image?

Due to the large amount of emails i'm sending with GMAIL i decided to
automatize this process using a script and following this
tutorial.Tutorial: Sending emails from a Spreadsheet
The "Message" is being generated by another function i created, called
prepareEmails.
The problems are the following:
1) How can i tell prepareEmails to add my personal signature? I can't
simply copy its text into that function, because my signature contains an
image (for which i have the URL), and i want that image to be into the
signature.
2) How can i make my signature BOLD?
Thanks everybody