Watermark can already be added automaticly. Inside admin panel you have to go to images upload and from there in bottom you got "Watermark" change it from disabled to enabled, then upload watermark logo you want it to add.
Obviously you can't read English very well, as it clearly sates in my post, watermark an image AFTER upload, in the same way we can mark an image NSFW AFTER upload, perhaps this will help,