You can create a signature that is appended to the body of your email message. You can create a different signature for each of your email identities. See How Do I Set Identity Settings for my Local or External Accounts? for more information.
In the Navigation pane, click Options.
Under Mail, expand Identities.
Select the identity for which you want to create a signature.
The Identity Settings for the selected identity appear.
Enter the signature in the rich text editor. Your signature can include a JPG, GIF, or PNG image file.
Select Add the signature to every message you compose.
Click Save Preferences.
The signature will appear in the message body for all emails sent from the selected account.