steghide is a mature GPL-licensed CLI tool for hiding arbitrary data inside of of image files (and some archaic audio formats). Its official web presence is located at https://steghide.sourceforge.net/, but it is ubiquitously mirrored in various repositories and package managers (. If you use APT, simply install it with
# apt install steghide
Until there is Nothing left.
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Contact: nihilist@nowhere.moe (PGP)