Images on WP Network’s subsite appear only if accessed via main site’s url

The problem is that thumbnail addresses are coded on the db. When changing a wp installation from one site to an other, you need to make sure that the thumbnail addresses are correct, one way is to use wp-exporter and importer I personaly don’t like that option what I normally do is download the entire db, then on a text editor run a search and replace and change my site that is telling the text editor to change something like: 'http://localhost/blabla/wp-content/' to 'http://www.misite.com/wp-content/blog.dir/8' this is the only way I know that a site can be moved with no problem try also search and replace the root path of the site like 'http://localhost/blabla' then just upload the db may be a little tricky since it is a multisite don’t know the exact probrems you will find, but I think the answer is on the db addresses of the thumbnails. Hope this helps.