In order for a SimpleViewer gallery to function correctly, all your SimpleViewer files should be located on the same domain or subdomain due to the JavaScript same origin policy.
If you are storing your gallery's XML files on a different domain or subdomain to the other gallery files (which you are if you change your uploads folder from 'domain.com' to 'img.domain.com'), then this is likely to be the cause of the problem.
If it were only the images that you were moving (and not the XML files) you could try using a 'crossdomain.xml' file as in this FAQ:
When loading images from a different domain, the images don't load or I see a 'Security Sandbox Violation' error. How do I fix this?