In reply to Dave 88:
paint.net +1 . Have used it for years. Imports and exports to just about any format via plugins and natively. Fast and easy to use. The only criticism is that plugins are not obvious to install - thats because its dead easy. Unzip the download files and copy the dll into the plugins directory in the program files/paint.net directory then restart.
Gimp is an "image manipulation package" (err Gnu.I.M.P) and not really a drawing package, although it can be used very successfully for this and it has many fans. I find it crashes loads on windows and excellent features get abandoned quickly. Gave up on it years ago, then tried again recently, and gave up again. The opaque menus and horrible dialogue boxes don't help.
If you use a pen and pad combi to draw then gimp does do pressure related brush widths/colour density which I did like, not sure about paint.net for this as never tried it.