While a program like Inkscape excels at editing vector graphics, a program like GIMP is best suited to editing raster graphics. GIMP offers several advanced tools, plus all the familiar ones for drawing, painting, cropping, color-picking, and bucket-filling. The image can be manipulated down to the level of individual pixels. GIMP is great for working with photographic images or the rasterized output from a 2D program like Inkscape or a 3D renderer like Blender.
GIMP stands for GNU Image Manipulation Program. It is one of many great pieces of free and open-source software brought to us by the folks at GNU.