Visual illusion

I've had a bit of fun today being totally puzzled by the vanishing leprechaun visual illusion, dating from the 1960s. If you cut the top half of the picture below into three pieces and swap the two upper parts, such that you get the lower half of the picture, you'll find that one leprechaun appears to have disappeared.

An explanation of the illusion is provided, with some building of suspense, on this website.
The Guardian recently had a piece on a variations of this puzzle, including one with sixties pin-up girls.

I've actually also been looking at more serious stuff today, namely MIT professor Agustín Rayo's plea for localism, which I hope I'll be able to come back to soon in a separate post, since it provides an original account of the Liar paradox. I have a hunch that what connects the pictorial illusion and Rayo's ideas is that context is everything.