DOC: Do not use Voronoi in Image pixel description

Per discussion:

  https://itk.org/pipermail/insight-developers/2018-March/025315.html

Image converted with

  fig2dev -L eps XFigOriginals/SoftwareGuide/Art/ImageOriginAndSpacing.fig ./SoftwareGuide/Art/ImageOriginAndSpacing.eps

Author: thewtex