Basicly I need a straight and simple free software gallery with this specific thing - it needs to have authentication for every user (so no photos are show before logging in) and it needs to have multiple user levels (set by admin). Every user level can acces only a limited set of photos. It would be also nice if upload would extract EXIM data and automaticly generate multiple sizes of photos.

All photos would bear caption and multiple set of tags / keywords.

All uploads need to be approved by the admin who also sets the user levels.

The final part of this app would be done by myself and would include using network MAC addresses as part of the intranet solution for a fairly large non-profit I am working right now...

So is there any possibility that this kind of script would exist already or should I sit down and do some programming ;)

I think Coppermine Photo Gallery has everything you're looking for, and it's free.

does it use dynamic file link ? so if some one hotlinks to it , it will check if he/she has a login name or not before he can see the file ?

edit: i just found out
http://coppermine-gallery.net/demo/cpg13x/albums/food/fruit/763_O.jpg
does not have dynamic linking so it will not have a good hotlinking option