Automatically watermark images as they are uploaded to the WordPress Media Library using Both Images and Text.
Adds a okay watermark (whether image or text) to images as you upload them. Even though it says you can add watermarks to previous uploaded images it only does that if you want to pay a $40 fee per website. They also have some other watermark plugins like Transparent Watermark, Bulk Watermark, and then the Signature Watermark, all cost separately and only have half the functions, they claim to have, for free.
I give only three stars because it's the free version of a more complete plugin.