Clicked Creative

Plogger
Plogger is the next generation in open-source photo gallery systems. A web application not bloated with superfluous features or complicated configuration settings. Plogger is a simple yet powerful tool — everything you need to share your images with the world.
http://www.plogger.org/

Gallery 2
“This is the official website of Gallery, the open source web based photo album organizer. Gallery gives you an intuitive way to blend photo management seamlessly into your own website whether you’re running a small personal site or a large community site. “

http://gallery.menalto.com/

A great article showing you how to get around the problem of png not working properly in Internet Explorer
http://redvip.homelinux.net/varios/explorer-png-en.html