I don`t know anything about netscape composer, but this is how I did it with Front Page.
<a href="mailto:email@example.com"><img border="0" src="maila.gif" width="100" height="32"></a>
mail.gif is the image to click on to start Outlook, or Outlook Express. The numbers define the dimensions of the image.
ps: I hope this will come up correctly when I click on Post response. If not click here to see the code: