My girlfriend drew the below figure in pen by hand, to be used as a cover for a scientific assignment. We then scanned the drawing to as high resolution as our crappy desktop scanner/printer would allow and I processed the drawing using the free raster graphics editor GIMP. I mainly retraced all the pen strokes, removing some, but not all, of the imperfections in the process. Added vibrant colors to all the of elements  before applying gradients to all of the individual colors to obtain the final result.