This App combines two images into one.
The two images are, however, NOT MIXED UP in the combined image.

What does that mean?

In Screenshot (1), you may see a portrait of Albert Einstein from a normal reading distance. However, from a few METERS away, you may see Marilyn Monroe.
So, the two portrait photographs are not mixed up. They still coexist separately in one image.
This is an example of HYBRID IMAGEs.

/// TRICKY PROFILE PHOTOGRAPH ///
Scaling down a hybrid image has the same effect as viewing it from a few meters away.
Assume that the hybrid image shown in Screenshot (2) is used as your profile photograph for a social networking service. Those who see it as a small icon in the time-line may think you are a beautiful woman, but in your profile page, where the image is enlarged, they will see a panther.

/// SECRET MESSAGE ///
In Screenshot (3), a cat says 'I am so hungry'.
However, his message is not visible when it is viewed from a distance (or scaled down to an icon size). So, you can embed secret messages in a hybrid image.
[ note: The message to embed needs to be prepared as an image by using other Apps]

/// IT IS NOT EASY ///
Honestly speaking, it is not easy to make an impressive hybrid image.
By trial and error, you have to find optimal conditions where hybrid images are synthesized as you expect.

/// LAST BUT NOT LEAST ///
The concept of hybrid images was originally proposed by cognitive psychologists Philippe G. Schyns and Aude Oliva. A hybrid image is synthesized by combining high spatial frequency component of one image and low spatial frequency component of another image. Psychologists often use the hybrid image as a tool to investigate visual perception/cognition.