When you visit someone's picture and spot yourself or you just want to say " Hey, I have been here" you may add a name tag to it.

Just visit the photo and you will see your "name tag" already visible on the top left corner of the picture.  Use the mouse to drag it to the desired place on the picture.  Make sure yo do not cover any existing tags!  Once that you positioned your name tag click on the "tag" little image under the photo. 

You can remove your tag anytime.