Is it possible to add visible/invisible watermark to photos already uploaded to Picasa Web/500px/Flickr?
Do they preserve EXIF(metadata) of the image?
I am looking for free tools/services. If downloading software is required, I would prefer Linux/Ubuntu.
Update: In the light of two answers already given; adding a clarification. A solution that lets adding the watermark without downloading the photos from the respective locations is asked for.