Automatically Logging in Users

Hi, I have a website where I have a lot of registered users. I want these users to be able to login as "members" (able to upload photos, unlike unregistered "guests") without ever having to type in login info. I was wondering if there is a way that they could be logged in by the http address, something like http://mysite.com/gallery/index.php?user=member&password=memberpass

If you have any other ideas on how to accomplish this, short of merging sql databases, I'm open to suggestions.

Thanks
Dub