(OT) What are these changable pictures called?

This is completely off topic, but maybe someone will know the answer.

Last summer at a fair, they were selling these pictures that change as you walk past them. For example. looking straight on, you see a collie dog by a tree. Step to the right and you see a small dog in a dog house. Step to the left you see a cat on a porch. (I just made up this example). But I hopefully am explaining what these pictures do.

There are like three pictures in one, and there is a plastic over them that somehow changes the angle of view, thus you see a different image as you move.

I want to buy one of these as a gift for someone who really wants one. I figure ebay has them, but I dont have the slightest idea what to search for. In other words, what the heck are these pictures called? What do I put in the ebay search bar?

I tried "three dimensional picture", that did not work. I tried "changing picture", that did not work either.

Anyone know?

Thanks