Each of the email sent from the system to your clients carries an email signature. The email signature can be edited very simply, and once updated, all future emails would carry the updated email signature.
Step 1: Select Email tab from the top navigation bar.
Go to Settings tab on Email. You will see an option of Email Signature, click edit to change the signature.
Note that if you use %%PROFILE-PHOTO%% , your current profile photo will be visible in your email signature. When the profile photo is changed the new photo would be visible here automatically.