From Wikipedia, "Steganography (/ˌstɛɡəˈnɒɡrəfi/ (listen) STEG-ə-NOG-rə-fee) is the practice of concealing a file, message, image, or video within another file, message, image, or video."
A short walk through of a few steganography challenges
A powerful, free, open source image editor that can be easily installed on Linux machines.
A common steganography program used in CTFs
A useful tool for quickly analyzing images, including a wide range of image manipulation tools
Binary analysis tool that allows hidden files to be identified and extracted