In my own system, Octo[0], I encode programs and their metadata in a similar steganographic fashion in GIF files[1]. As others have noted here, both GIF and PNG offer extension mechanisms and ways to embed "comments", but popular image-sharing sites universally re-encode images and discard this data. The advantage of GIF over PNG (for my purposes) is that I store an arbitrary payload in a fixed-looking image by creating additional frames of animation.
In the past, I've also used a different technique- if you simply concatenate a PNG onto a JAR (which is really just a ZIP archive) you end up with a file that acts like a PNG unless you change the extension to JAR, in which case it acts like a Java executable. This works because the PNG header is at the beginning of the file, while the ZIP header is at the end. Nowadays, though, desktop Java is pretty much dead, so it's a less exciting party trick.
> In the past, I've also used a different technique- if you simply concatenate a PNG onto a JAR (which is really just a ZIP archive) you end up with a file that acts like a PNG unless you change the extension to JAR, in which case it acts like a Java executable. This works because the PNG header is at the beginning of the file, while the ZIP header is at the end. Nowadays, though, desktop Java is pretty much dead, so it's a less exciting party trick.
It was a common party trick for the reasons you outlined on image boards: you could smuggle small-ish zip files as long as the board made the original file available as-is somehow by concatenating the image (usually jpeg) and zip.
In the past, I've also used a different technique- if you simply concatenate a PNG onto a JAR (which is really just a ZIP archive) you end up with a file that acts like a PNG unless you change the extension to JAR, in which case it acts like a Java executable. This works because the PNG header is at the beginning of the file, while the ZIP header is at the end. Nowadays, though, desktop Java is pretty much dead, so it's a less exciting party trick.
[0] https://github.com/JohnEarnest/Octo
[1] https://github.com/JohnEarnest/Octo/blob/gh-pages/js/sharing...