On the secondary sites, the images are not shown and a forbidden message is displayed.
This comes from these lines in the .htaccess:
RewriteRule ^wp-admin/includes/ - [F,L]
RewriteRule !^wp-includes/ - [S=3]
RewriteRule ^wp-includes/[^/]+\.php$ - [F,L]
RewriteRule ^wp-includes/js/tinymce/langs/.+\.php - [F,L]
RewriteRule ^wp-includes/theme-compat/ - [F,L]
which are not compatible with this one:
# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]
Do you have a fix for it ?