Thanks sarkut! I have installed the custombrushes mini plugin, and those PNG brushes ... but two things are bothering me. The plugin doesn't seem to integrate the custom brushes anywhere except in that special window that hangs off the Effects menu, and anyway my laptop can only render the "misty" brush in blobs that are trailing behind my cursor. Is the programming language of Paint.net too slow to implement airbrushes, and is that why they aren't a feature of the main interface?
Cheers, Ross.