The Digital Invisible Ink Toolkit is a Java steganography tool that can hide any sort of file inside a digital image (regarding that the message will fit, and the image is 24 bit colour). Digital Invisible Ink Toolkit will work on Linux and Max OS, even on Windows because it is written in Java and thus platform independent.
There are four highly customisable algorithms in the tool, as well as an open-source implementation of RS Analysis (an extremely good steganalysis method). The tool has the additional advantage of being able to simulate hiding - so you can get an accurate map of where the information is hidden.
The compiled version can be run by simply double clicking the .jar file, or by running at a command line with the following options (you will need to run at the command line if you are using big pictures, such as those greater than 500x500 pixels or it will run out of memory):
java -jar -Xmx512m diit_1-1.jar
Where -Xmx512m tells the virtual machine to use 512MB of physical RAM (at most) - please change to suit your own machine specifications.