GIMP

GIMP (GNU Image Manipulation Program) is a raster graphics editor. If you are looking for a tool to perform basic editing (cropping, rotating, resizing) then I tend to recommend Pinta or Shotwell. GIMP can perform these basic tasks but it can do a whole lot more.

GIMP, like most professional editors, has the concept of layers. This is especially useful if you're using reference material to sketch over or trace. There are plenty of tutorials you can follow to build up your basic skills and knowledge.

In my post Anime Waifu GIMP, you'll see how you can use GIMP to improve a hand drawn sketch.