You add a user picture or choose from available ions via System Preferences/Users under the Picture tab for your login account and this can also be done with your contact information in the Address Book.
Adding a user picture via System Preferences/Users is an easy process but removing it from there isn't as easy.
To remove the existing, go to your contact information in the Address Book. Select Edit to edit your contact information. Select the "Me" picture area to highlight it and then the Delete key.
The user picture or logo selected is not sent with your messages. In order to do so requires storing the picture or icon on the .Mac server (if you have a .Mac account) and even then, it would only appear if the recipient is a Mail.app user.