Steganographic key ring
A quick search reveals a range of steganography software and solutions for Mac.

The following all appear on the first page of results:


iSteg
SilentEye
Stegosec


As @patrix mentions, security through obscurity provides little security. Be sure to encrypt files and information being embedded in images first.

Tools exist for mass checking images for embedded information and alterations. Search for an image potentially containing embedded information is a task well suited to those with plenty of computer power.