From the message creation screen, select the image you wish to replace. Do this by clicking once on the image in the email. It will be surrounded by a black frame. Click on the tree icon for insert/edit image. On the resulting Insert/Edit Image page, paste the url of the new image into the field for Image URL. Click Update.
If you are not hosting your own images, you will need to have already uploaded the images you wish to use in your email into the Mustang List Image Directory. (See KB article for detailed instructions on uploading images.)