I know I'm coming late to the game; I visited this thread just to reconfirm the original instructions about copying onto a USB stick from a "normal" installation.
I am another software developer who loves Paint.Net, so maybe I can clarify some things and set people's minds to rest...
Rick (the author) clearly states Paint.Net relies upon Microsoft's .NET framework.
This has several implications that will not be obvious to non-developers...
The .NET framework has a LOT of functions that make it easier to write something like Paint.Net. This is probably why the author used it!
Microsoft designed the .NET framework to pretty much invasively "wire itself into" Windows itself. It's not a library you can just plop on a USB and have available on any PC. It must be installed on the PC you're going to run .
Therefore, Paint.Net can never be truly and "fully" portable. You can carry it around on USB, but it cannot be self-contained.
To make Paint.Net completely self-contained and fully portable would require one of two things to occur:
Rick could write his own local versions of all the .NET functions he's used. This would be extremely time-consuming, and error-prone, and would make the application much, much bigger in terms of file sizes and memory requirements.
He could also completely rewrite Paint.Net to be self-contained. This might actually be easier than writing his own versions of the .NET functions! Which is still time-consuming, error-prone, and makes the application much bigger. Consider how long he's been working on Paint.Net --- he'd have to be out of his mind to decide to do a complete rewrite, but even if he decided to do so out of complete kindness, perhaps you can now imagine the scale of the work this would be asking!
This limited portability is Microsoft's miscalculation, not Rick's. Microsoft assumed that everybody would have .NET on every Windows machine, and that those would all stay updated. That actually has mostly -- but not entirely --- happened. But... some people or organizations will always be behind the curve. And since Microsoft keeps improving .NET and increasing the version numbers, the target PC must be running at least a certain level of .NET so that the functions called by Paint.Net would be present.