Themes don't find files after mapping

Hi,
I've installed Domain Mapping and it works ok (mapped externaldomain.com to this.internaldomain.com).
The only issue is that the Theme is looking for files on the mapped url - and getting 404s. E.g. the logo is in internaldomain.com/wp-content/themes/images but the newly mapped site looks for it at externaldomain.com/wp-content/themes/images - and it's not there.
I feel it might have something to do with nginx, but not sure.

Thanks for the help!