What are the Tibetan sketches in the movie Samsara called?

In the movie Samsara by Pan Nalin, an older monk shows some sketches to the protagonist monk. The sketches are erotic in nature but when held in front of flame reveal true nature as skeletons. It's a very powerful teaching. Do these sketches have any significance? What are they called? This is the youtube link for the movie (look @44:37). Screenshots from the movie are copied below -- they are hidden using 'spoiler' tags because they are "erotic in nature", so mouse-over the image if you want to make them visible. >! And after changing the angle of the 'paper' (there's light from a candle on the other side): >!