Paint.net is portable and it works great. You can find it at the following link. (in English)
http://framakey.org/Portables/PortablePaintNet
If you look hard enough you can find this information in these forums but I had a dificult time finding it so I thought I would start a new topic, to make it easier for people to find the information.
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Unfortunately, that's an outdated version as Paint.NET is no longer open source. Plus, it requires .NET which means it's not really portable except on Vista and Windows 7 or XP boxes you can install .NET on.
Sometimes, the impossible can become possible, if you're awesome!