Here’s a great free script that I was able to get working easily on one of my sites:

Watermark Images

This script will take any image and automatically add a watermark before it is delivered from the server to the person’s browser.

• Covers all the image formats nicely: works with .jpg .gif or .png file
• Watermark image is a basic gif file, no fussing with fancy png files

I haven’t had a chance to dig around the site yet, but it looks like there are plenty of tips and script goodies in there, something for everyone. Love that!