Creating watermarks

I've been learning how to use PHP with the GD Library and I've managed to learn quite a bit. I can upload, resize, create thumbnails and I'm even able to create "security code" images for forms. My question is how do I create a Watermark on the image? I want something transparent but still visible enough to see the copyright and the website. I want to put it in the center of the image. I would use a non-water marked image and when it's called on (depending on viewing size) I want the water mark to be added. How would one go about doing that?