I have been browsing around for something along the same lines-- I'd like to be able to allow multiple people (family, friends) the ability to add information to the database for each picture, using only a browser interface. Optional usercode/password to have modify permission. Subsequently I'd like to be able to search for words associated with a picture.
Seems like the critical mass is there in any of the main distibutions to do the job: your choice of Postgresql, MySql for databases and Apache to serve it all up. What is needed is the glue to put it together.
A portion of the answer may be the "My Photo Gallery" Perl script at
Free Perl Scripts
which also has free perl scripts for doing classified ads using SQL for data access.
Perhaps there is also something to be found at a couple of the photography websites, which themselves offer a database for a fee.-- photo.net and the discussion of its construction by Phil Greenspun is one; fredmiranda.com is worth checking out too.
hope this helps, & will look for additional suggestions to be posted.