The hybrid image illusion is a pretty simple optical trick. It relies on the fact that people see different things in an image depending on how close they are to it. This illusion allows people to see one picture close up, and another from far away.
Albert Einstein and Marilyn Monroe look nothing like each other, but there is a way that their faces can be popped into the same image, with one face flowing into the other. The only thing required for the flow is a set of human eyes and the ability to move closer or father from the screen.
In a hybrid illusions, one image involves fine but dark black lines tracing out the line image of, in this case, Einstein's face. His face appears on a fuzzy gray background. From close up, it's easy to see the dark lines, but each part of the gray blur isn't too easy to pick out from any other part. For the other face to emerge, just get up and move away from the screen. (Or shrink the image down.)
As you get farther from the picture, it comes together like an impressionist painting. The blurry blobs pick out an actual shape. Meanwhile, those fine black lines aren't as easy to see anymore. As the eyes become unable to see the details, but more able to define the shifting shades of the larger picture, Marilyn Monroe's face is clear on the screen.
This is one of the simpler illusions. All it takes is two things that share enough of the same shape that they don't look two weird when layered on top of each other, and the will to blend them. You can see this image, as well as many others, at the MIT Hybrid Gallery.