I've had that problem myself. For me, the problem was the .htaccess file.
In cpanel, go to File Manager > Public Html > .htaccess ... Once you have the .htaccess file open, check to make sure that all of your subdomains and addon domains are listed in every possible variation (i.e.
www.yyy.com, yyy.com, aaa.yyy.com,
www.aaa.yyy.com,
www.bbb.com, etc) always making sure you have the appropriate notations following (I just copy and paste a set of urls that are already there and fix the url to what I need.)
Make sure any blank spaces at the bottom of the page are deleted, and save the file. That should give the addon domain permission to use images and files.
Also, if you have any kind of image blocking turned on to keep people from linking directly to your images on your site, make sure you add the addon domain to the list of allowed sites, so the server doesn't block access.
Hope this helps.
