Save for Web Automation.

This is an automator script saved as an application for converting images to a web-ready format. Download and unzip the application, then drag your photos onto it: the converted photos will be placed on your desktop. Inspiration for this program can be found on Friendfeed here. It should work in Mac OS X 10.4 and 10.5: hopefully it works for you!

Special note: It saves all files on the desktop, and if it sees a file on the desktop with the same name, it won't overwrite that file. So, make sure you keep your originals somewhere other than the desktop (like maybe in a folder).