Thanks for your reply. I want it to be a comment for myself, not necessarily hide code or secret messages. Something like comments in LibreOffice or even PP and similars.
First thing I found in a quick search: https://manpages.ubuntu.com/manpages/jammy/man1/stegosuite.1.html
Discussion
I don't know of any way to do that, sorry.
You might want to look up some kind of arbitrary file-tagging software. Something that basically adds a "sidecar" database of arbitrary comments/tags to filesystem references.
Broaden your search from "images" to "any files on Linux" and you'll likely have more luck
Some (admittedly daunting and weird) ideas here: https://stackoverflow.com/a/68718493
Thanks! I just now saw this, didn't get a notification in #Coracle. Or maybe they expire after a while, I was traveling, dunno.