I've had a few requests for Paint.Net tutorials. I've basically learned some of what I do by trial and error and having my husband look over my shoulder saying "why'd you do it that way?". I've looked around on the internet and here is a place that might be of some help to you.
They have lots of tutorials describing different things. They are called Lunartics.
Here is a tutorial that shows how to whiten teeth in Paint.net
Here is a video tutorial that shows how to make fire in Paint.net
Hope this helps some of you get started. It really is an amazing program, has a fairly short learning curve but because of how powerful it is it's not super-easy.