PNG and Hidden Pixels
Over the last few months, I've been seeing an increase in a third type of hidden pixels: PNG padding. Almost all of these sightings have been associated with steganographic challenges, capture-the-flag forensic contests, and similar puzzles. (Ever since people began staying home due to COVID-19, there seems to have been an increased interest in security, steganography, and related topics.)
Source: PNG and Hidden Pixels, an article by Dr. Neal Krawetz.