When left in a comment under a picture it indicates that a particular picture on a social networking site (like Facebook) was taken by you. Only used in circumstances where it isnt apparent.
Guy 2: Yeah, but I commented ittp under it so everyone knows who took it.
2.) Guy 1: Doug finally posted the pictures from the last trip to the zoo. That one you took with his camera of the monkey jerking off was priceless.
Guy 2: Yeah, I better put ittp under that one so Doug doesnt steal all the credit.