Did you see what picture you have set in your Address Book application? Open up Accounts preferences, then click the Open button next to Address Book card. To remove the picture, click the Edit button under the card, and click on the picture, hit the delete key. You can also substitute another picture if you prefer. For future reference, the picture is not actually included with outgoing emails, that's just a quick at-a-glance method for you to see who sent the email.