Storm-Engineer said:
There is actually a whole field of computer science called Steganography that deals with the concealment of data inside other data, what orange explained is one of the basic examples of it.

Depending on the details the result may be an image file that has artifacts as if it was a damaged images (like when a JPG has part of it missing and pure black), or simply an image that appears completely normal, but unusually large file size - in case of large data. If the data is small it may be impossible to know it's there until you look inside the file.

This is actually one of the methods used for tracking files (eg images) online.

And of course also used by malware but there it gets more complicated as it's not enough to hide the data there, you need to get the target computer to execute it unknowingly.

Yeah this isn't anything new. I remember "Stitching together" images into one file yeaaaaars ago just by hex editing them in the right places. Made it easier to get stuff up onto BBSes before archives became a common thing. Was also handy if writing a game to stick any info you might want to pass to or from the game directly into the sprite.