If you’ve noticed a picture of yourself in emails you send and would like to keep things a bit more anonymous, don’t worry. The source of that picture is Apple’s Address Book application. When you open Address Book and choose your contact card (Card -> Go to My Card), in addition to your contact information you’ll see any image you’ve assigned to your contact.
Note that your picture is not being sent across the great expanse that is the Internet as an attachment. Others who receive your mail won’t see this image. Rather, you’re seeing it because it’s part of your Address Book card and Mail is displaying it because Mail displays pictures assigned to contacts in your copy of Address Book. Those you send messages to don’t have that image in their copy of Address Book (unless you’ve given them the image and they’ve assigned it to your card) and so it doesn’t display in their copy of Mail.
If you no longer wish to see the image when you look at your own messages, just open Address Book, select your card, and choose Card -> Clear Custom Image.