PostgreSQL need to be configured properly before you can use GNOME Photo Collector correctly. Some of the following steps may be executed by tools you can use to install PostgreSQL packages, but I already found Linux distributions where PostgreSQL was not configured correctly :-(
if not already existing, create a password protected user named "postgres".
login on your system, using this "postgres" user.
create the database which should store GPC informations, with "createdb dbName" (but as GPC database objects names are always prefixed by "gpc_", they are probably uniques, so you could also use another already existing database...).
launch GPC for the first time on this new database, with the "postgres" user ; on the first connection, GPC will automatically create required tables.
if available, launch the "pgaccess" tool, which provides graphical tools to administer your PostgreSQL database.
in the "Users" tab, create a new DATABASE user with the "New" button ; this user can have the same name as your login name (and generally, it should have !).
in the "Tables" tab, each table which have a "gpc_" prefix is a GPC data table ; for each of these tables, use the "Design" button and then the "Permissions" tab to grant access to selected users to these tables ; the "PUBLIC" user rights are used for users not listed otherwise in the list.
logout ; from now, you should be able to login, open a connection to your PostgreSQL database and use GPC with your usual login name.
If you can't go completely throught this process without errors, you should verify a few things :
does your "postmaster" process runs with the "-i" option, which enabled network connections (even from your local loopback device !) ? If not, check your PostgreSQL configuration file or the script used to launch postmaster.
can you connect to your database using "psql -d dbName" (or "psql -u -d dbName" to enter username and password) ? If not, check again your PostgreSQL configuration and starting scripts, and check PostgreSQL log files.
If you still have problems with your PostgreSQL connection, please have a look at the man pages for "createdb", "createuser" and "psql", which are the basic PostgreSQL administration tools ; "pgaccess" is also a graphical tool (written in TCL) which can be used to handle your PostgreSQL database easilly.