1. Click on your name at the top right of the page > select My Preferences
2. Then select: Email > Signature Options
Note: Even though this in listed beneath 'Email' options, the same code is used to populate this image in merged documents like SOA and FDS.
For a guide on setting up your email signature, please refer to the article below;
How to set up your own Personal Email Signature?
Published by Garrett White