The guy who originally found it, Johnleekl of the TMF, found it here, a site mostly filled with very adult hentai. The person who posted it there did not specify as to where he got it in the first place.
Sorry, but I have to disagree. It looks more like an innocent rub of a veeery ticklish-footed girl! Notice the thumb is pushing against her foot rather than rubbing with the tip. Maybe a very poor tickling technique, but still a cute pic.