The picture you're seeing is the account picture you have set for yourself; it doesn't actually get sent with the email. Because of that, you shouldn't need to worry about removing it. (This is simply so that you can identify the sender of message; if you receive a message from someone whose email address is in your Address Book, and you have a picture set for that person, you'll see his or her picture as well.)
If you do want to change it, though, just open System Preferences, click on the Accounts pane, choose your account, and then click the image and choose a different one. You can also set it to any picture on your Mac (or take a new picture of yourself) by choosing Edit Picture… from the menu that opens after clicking your account picture. You can also change your picture from within Address Book.