You can also glitch bitmaps in Notepad++, or a Hex editor. I copied and pasted some text around, added and deleted some random characters, and viola!
PNG is an incredibly stable image format. You can append a long message on the end of a PNG, and it won't break or look visually different. It's like an easter egg for people who do things like opening images in text editors, and I've seen this used for both fun and for nefarious reasons in the past. Download and open this PNG in a text editor, and scroll to the end:
https://files.catbox.moe/a9635c.png