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.
By importing external images I mean making and keeping local copies of all the linked images in posts and pages, adding them as WordPress gallery attachments of your WordPress site.
Now you can adds a watermark signature or image to all of your uploaded images in your WordPress site. Signature Watermark plugin uses PNG images with transparency for watermarking your site’s images.