PicMarkr lets you to add custom watermark (image or text) to your images online and free. It is useful when you need to protect your copyrights or if you want to add comments to your photos. It is easy to use with minimum settings, no need to download or install anything. You simply need to upload files from computer or grab them from your Flickr account. It also allows batch image processing, it can saves you a lot of time when you are adding watermark to many images.

Website: http://picmarkr.com/