This might work...

First back up your newsletter file, then convert a copy of the file to a Web
Publication (File > Convert to a web Publication). Then Insert > HTML Code
Fragment. Insert a code fragment that links to where you have the image on
your website:
<IMG SRC=http://www.yoursite.com/images/yourimage.gif">

This assumes that you have an image called yourimage.gif stored in a folder
on your site called 'images'.

Then resize your code fragment box to the size of the image and move it to
where you want the image to appear. One way I do that is to insert the
original image into the document, then move the code fragment so it Snaps To
the image, I then drag the width and length of the code fragment box until
it Snaps To the size of the image, top and sides. When you are done, delete
the image that you inserted, and move the code fragment box to where you
want it to appear in the document.

You will probably have to do some adjusting, so email it to yourself a few
times until you are satisfied with how it looks. You may find that you can't
use a master page, tabs, word wrap and several other paragraph formatting
options that you use in a print document.

DavidF