hi,
Paint.NET is natively not a drawing / painting tool (in its default install stage), I would dare to say. its rather a simple image editor. however it can by widely enhanced with tons of free downloadable plug-ins.
I personally use Paint.NET a lot and my "magic" keys are F5, F7 and F8 there
but all my paint art is made with vector editor called
Inkscape, as SVG project layered, exported then into finall PNG file. this way I can easy back-edit any line, shadow or whatelse there, always and also export ito any image resolution, oh yeah!
in case you mean just to
edit skins or creating specular maps or bumpmaps from existing ones, and/or
save the final stuff into DDS files, then Paint.NET is really a pretty good choice. however to paint / create skins from scratch would be bit complicated so I personally would reccommend to use another bitmap paint editor (GIMP, PaintShop Pro, PhotoShop...) or some of the few vector based ones.
anyway how to use Paint.NET just google