# Image interpolation_mitas_mitasova.gif
http://skagit.meas.ncsu.edu/~helena/gmslab/asae97/hasint.gif

=> It is cited with the source in the image subtitle: 
Mitas, L., Mitasova, H. (1999): Spatial Interpolation. In: P.Longley, M.F.
Goodchild, D.J. Maguire, D.W.Rhind (Eds.), Geographical Information Systems:
Principles, Techniques, Management and Applications, Wiley.

# Image delaunay.png
http://upload.wikimedia.org/wikipedia/commons/a/a7/Delaunay_circumcircles_centers.png

=> This image is licensed under GNU FDL.
http://en.wikipedia.org/wiki/File:Delaunay_circumcircles_centers.png