pngarc is a utility for hiding files inside PNG images (and getting them back out again)
How this all works

pngarc 0.3 python (multiplatform) source (8k)
pngarc 0.3 win32 binary (750k)

Usage: (If using the windows version, just leave off the .py)
To extract files from a PNG:
pngarc.py x file.png

To list files inside a PNG:
pngarc.py l file.png

To add a file to a PNG:
pngarc.py a file.png file_to_hide.dat
Files inside PNGs are not compressed, so zipping them before adding is a good idea.