What are the images called when in one picture you can see two different things?

Question

For example lets say that there is a portrait of an old women, but when you look at it different it becomes a young women, even though it's the same picture.

If someone could help me it would be great!!

Answer 1

These are called double image paintings. See examples in Dali's work at http://www.cwru.edu/artsci/engl/VSALM/mo...

Answer 2

It's called Optical Illusion

check here

http://en.wikipedia.org/wiki/Optical_ill...