Drupal sites with multiple domains
Drupal 6 has a problem with running a site on multiple domains. The files directory is typically set to /sites/$domain, which is fine until you start accessing a site using a different domain. The resulting mess depends upon which modules you are using to serve your images, but an example url that might be generated is /sites/example.org.uk/files/sites/example.com/files/1.jpg and this obviously does not exist in the file system.
If you've moved the site rather than want both domain names to be valid then it's easy to fix
UPDATE files SET filepath = REPLACE(filepath,'sites/example.com/files/','sites/example.co.uk/files/');However to get both working requires some jiggery pokery with symlinks. The exact urls generated by Drupal will depend upon the module you are using and the domains you are wanting to serve from, but ln -s is your friend. Imagecache is the worst offender I have come across so far, with symlinks needed for each size of image, however other modules (and themes) will handle things differently.