imgi Image Host is a free, simple and easy-to-use Image hosting script, which aims to make it easy to set up and maintain a photo/image hosting/sharing site.
Coppermine is an easily set-up, fast, feature-rich photo gallery script with mySQL database, user management, private galleries, automatic thumbnail creation, ecard feature and a template system for easy customization to match the rest of a site.
jp-gallery is a user-friendly gallery script on web-site. It's requires PHP5 (with GD-library) and turn-on Javascript on the client's browser. Optionally Mysql is needed - to enable photo-comments.
yapps (yet/why another php picture script) is just what the name tells you. Simple picture script/gallery that's using convert and montage from imagemagick to create thumbnails. Reads almost every data from a mysql database.