YASA will take an source image in bmp, png or jpg format and hide any type of data inside of it and save as png
http://yasa.hollow-net.net
EDIT:
Tested YASA under Ubuntu 7.04 using Mono and it works perfectly fine.

If a mac user with an intel and/or ppc cpu could please test using mono it would be greatly appreciated.
Ubuntu 7.04 running YASA








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would be great also 