Generally, JPEG images make for poor signatures.
A) not everyone has their mail client set to download/display images
B) it adds bulk to every message
C) it isn't clickable, so contextual-aware parsers can't 'read' the image to create a Contacts card, or ctrl-clicking the address won't bring up a map, ctrl-clicking an email address won't draft a new email, ctrl-clicking a phone number won't call you, etc.
That said, Mail.app's handling of HTML-formatted signatures leaves a LOT to be desired.
Often, pasting the signature into the Signatures preferences gets you most of the way there, but sometimes you need to edit the HTML directly to get it to do what you want (especially if there are inline images/logos that you want to use).
Additionally, Apple don't make it easy to find just where the signatures are saved on disk, and it varies for different OS/Mail.app versions.
For recent versions, check ~/Library/Mail/V8/MailData/Signatures you should see a number of .mailsignature files, one for each signature you have configured in the app. They are basic HTML (but without the normal HEAD/BODY tags of a web page). You can view these in any text editor and make changes (after making backups, of course :) ).
