Fjumper

It sounds like a good start would be to ask for a run down of Alpha Masking in general.

I've been using paint.net for a while, and have got the beginner's basics covered fairly well (layers, some effects, etc.). I'd like to make the jump to intermediate user, and start using and understanding things like alpha masking, other key pluggins, and the more complicated effects and tricks. When I stumble across something interesting, this forum is great for finding tutorials on that particular topic. The hard part is knowing where to go next. If you've never heard of alpha masking, for example, you'd never think to learn more about it or how it could help. Can anyone recommend a list of must-know topics that fall between "complete newbie" and "experienced user"?