Steganography

Hide a secret message inside an image's pixels, or reveal one that was hidden.

Hiding data in the least significant bits of an image works because changing the last bit of a colour channel shifts it by one value out of 256, which no eye can detect. Spread across enough pixels, that gives a surprisingly large hidden payload in a picture that looks completely ordinary.

It is concealment, not encryption, and the distinction matters. Anyone who suspects a message is there can extract it, and statistical analysis of the bit distribution can reveal its presence. It also does not survive re-encoding: any lossy compression, resize or platform re-upload destroys the payload, so it only works with a lossless format kept intact.