That is a function of the Text/HTML browser of your website.
We actually suggest you create the HTML in something like Dreamweaver. Then just paste it into the browser. Or use the word send method.
This actually is the exact reason we created the ability to send messages created in Word. It includes the images.
That all being said, all you have to do to fix this is after creating your email with the normal text/html editor of your site, is change to HTML view and change the relative URLS to the full path.