Picmarkr is a free web tool that will help you to watermark your images. It is very simply and easy to use. No sign up is required for using it. Picmarkr allows you to water mark your Flickr, Facebook, and Picasa accounts.
After upload your image Picmarkr provides three types of watermarks text watermark, image watermark and tiled watermark. After finish your work you can download the watermarked image into your computer easily. Files must be 25mb or less and watermark up to 5 images simultaneously.