mojoPortal will automatically resolve relative urls to fully qualified urls in the newsletter
To figure out the problem with the images you would need a way to inspect the markup actually delivered in the email to see if the url is correct for the image.
For example if your site is on the internet but you are also accessing it directly from the server as localhost and you send the newsletter from that context it would cause this problem because the image urls would have localhost in their urls not the public internet address, and localhost always points to the user's own machine so the image would not exist at that url. You should login to your site from the public side to send a newsletter.
Hope that helps,