The only technique I could find which somewhat resembles the image above is steganography, which is when information is 'concealed' or 'hidden' within the bigger picture. So like at first, you may notice the images or the word Newtown, but after a closer inspection, you will notice the other. But once again, the technique seems like one not actually employed in the image above and is definitely one you may not remember, so I'd probably not use it.