Facebook recently changed their permissions model for applications requesting access.

http://developers.facebook.com/docs/authentication/permissions for information.

This means that gthumb facebook exporter is no longer able to upload to existing facebook albums, which is an annoyance.

To fix this, some things need to be changed. Gthumb needs to be fixed to request the correct permissions.

In facebook-connection.c, method get_access_type_name, you need to append "user_photos" to the requested permission set.

The user, once using the fixed gthumb, needs to remove gthumb from the authorized applications list, then re-authorize with the fixed gthumb, at which time facebook will allow upload to existing albums.

Hope this helps