Here's the code. Just copy this line and paste it into your message where you want the picture.
<!--#enclosure filename="<name of enclosure>"msgNum="<message number that contains the enclosure>"-->
Then Replace name of the enclosure with the filename of the picture such as Dave.jpg.
Finally Replace message number that contains the enclosure
If you have only one enclosure just use <!--#enclosure msgNum="30"-->
Be sure you use the message number of the current message. you should be able to identify it in the browser address bar. Read more about doing this here:
Images in your messages
Here are some images I've uploaded already. You can use them too. Each has the code to make it work for you right below it. Pictures