A very capable alternative to Paint.Net is PhotoFiltre; both are quite handy and can easily do lots of basic editing. XnView and Irfanview can do basic editing, but it's all too obvious that isn't where the focus is.
GIMP is in a league of its own; not Photoshop by a long shot, but far beyond anything the above crop can do. If it's ever given a total revamp by a real UI designer (and they skip the interminable load time for font at startup) it'd be a one-stop shop for all things image.
But somehow, in the desktop world there's just no equivalent to the instant-tweaking editors of the mobile world (Snapseed, Instagram, etc). Multiple times I've been upset that Photoshop, let alone all the free alternatives, makes it so difficult to do trivial things. There's still a long way to go in the editing world....