Here are three image water marking plugins that might full-fill all your image watermarking needs on your WordPress site. All three plugins are developed by the same developers so check them out and and see which works best for you.
Transparent Image Watermark
Easily adds a watermark to all uploaded images of your WordPress site using PNG images with transparency.
Signature Watermark
It adds a watermark image or signature or both to all of your WordPress site uploaded images, using PNG images with transparency.
Bulk Watermark
Simply adds an image or text or both watermark quickly in bulk to all uploaded images in your WordPress site, using PNG images with transparency.