Steganography is the art and process of putting one type of information inside another in an attempt to hide it. This is often done in situations where an individual wants to preserve secret information inside normal objects.
Steganography is not a new innovation. Since the time of ancient Greeks, historians have written about how cultures used steganography to hide information in plain sight. To that end, one famous account was Herodotus’s story of Histiaeus, where he tattooed a short message on a servant’s bare scalp. It is now possible to insert an entire book inside a simple JPEG photograph.
This guide shows five of the best steganography tools currently available in Linux: Steghide, Stegoshare, Wavsteg, Snow, and Steganoroute. It also shows you how you can hide your first message using these utilities.
Information can be concealed inside image, audio, text, and even TCP ICMP packets. And the image, audio and text files would still open and be processed like their original file types.
See https://www.maketecheasier.com/best-steganography-tools-linux/
EDIT 2023-09-04: A good Android app that hides messages inside an image, is PixelKnot: Hidden Messages by Guardian Project, available on the F-Droid store app.