This will be due to a problem with the URL (location of the file on a server) - is incorrect. i.e. “../images/header.gif” instead of http://www.youraccount.exvn.com/images/header.gif.
A common problem is that your image worked when previewed on your machine but not in the message preview. This means it is stored locally, please make sure it has been uploaded and the correct URL has been used.