Since I am spending my holiday currently I finally have time for my own tests. Few days ago I got an idea about hiding images inside other images (I thought about some old paintings that I have saw which achieved similar effect) and wrote a software trying to do that. Here is a collection of images from my tests with the software. The images have been cut from MUCH larger original images (the background images must be large to get enough possibilities) but no other hand-manipulation has been made.
Basically I just give the software a background-image and another image which should be hidden. The software seeks through the images and tries to find a best possible position for the image to be blit. The actual blitting uses some interesting techniques as well; it calculates the errors between target/source colors and manipulates the final pixel according to that (lightness, color, local-contrast, etc. )... I'm not sure what this technique is called, maybe it has no name. I just call it "natural blend" :)
What do you think?