Thanks to SFI Affiliate Martins Gulbis for these instructions.
How To Create A Signature For Mozilla Thunderbird
If you are using Mozilla Thunderbird for your email program, you ad a signature by going to tools menu, selecting account settings, and then for the desired email account you check use a signature. You then have to enter the name of the file that contains the signature.
For a signature that includes a banner that the person reading can click on, you need to create a small html file for this.
For example, I use the following SFI banner:

And the following is the contents of the 'signature.html' file I use to create my signature:
<html>
<a href="http://www.gulbiscrafts.com/sfi">
<img src="file:///e:/sfi/banner12.gif"
alt="Check out my home business" width="125" height="125" border="0"></a><br>
</html>
This assumes that the image is in a file called banner12.gif in the sfi
directory of the e drive, and will send anyone who clicks to the SFI related
portion of my website. Most will want to use one of the SFI supplied
gateways.