I have it working on one of my forums, permissions generally set for "Can View thumbnails - Yes", "Can download attachments - No", but you can also do this on a forum by forum basis in permissions (make sure that the user group permissions for that section are not in conflict with the default settings).
On this forum I have download attachments enabled and view thumbnails yes
http://www.thamesvalleybirds.co.uk/bird-photos-general/
On this forum I have download attachments to no, but view thumbnails yes
http://www.thamesvalleybirds.co.uk/a...aneous-photos/
PS. multi posting in your own thread with the same message will not spur people to help you, most will just ignore it.