F5 is a free and open-source steganography algo that hides information in JPEG images. Unless other implementations it really hides it inside the image itself (not in metadata/comment fields or appended to the end of the file).
F5 hosts the source code for a Java F5 steganography implementation as open source.
Detailed instructions on how to use the F5 utility on your Mac are available HERE.
F5 is a cross-platform utility capable of running on any operating system that comes with Java support (e.g. Mac OS X, Windows, Linux).