How to generate protein graphics for illustrating articles?
All comments are great, there are numerous software programs that allow you to make such images. 



Gathering information about the insulin structure

By simply clicking on the image in wikipedia it will provide you with  valuable information: 


  Created by Isaac Yonemoto created with en:pymol, en:inkscape, and
  en:gimp from NMR structure 1ai0 in the en:pdb. Ref: Chang, X.,
  Jorgensen, A.M., Bardrum, P., Led, J.J.


So the creators used pymol, inkscape and gimp, but let's dig a bit deeper. Unfortunately Isaac Yonemoto, the editor of this wiki, did not provide a direct link to the original publication (see wiki file history). However by searching the author names in combination with insulin we quickly find the article: Solution structures of the R6 human insulin hexamer. In the first paragraph we see: 


  The atomic coordinates of the 20 refined hexamer structures and the
  NMR-derived restraints have been deposited in the Brookhaven Data Bank
  (PDB ID codes 1AIY and 1AI0 for the symmetric and the nonsymmetric
  hexamer, respectively)


You can find the structure that was depicted in this wiki article by searching the PDB id.



Creation vs visualization
You can interpret 'create' in your question in two ways here:


Creating this whole structure from nothing
Visualizing a structure someone else determined


If you refer to the former you should look at the article corresponding to this structure. Note that this is quite detailed and requires  knowledge of several concepts, such as molecular dynamics and NMR spectroscopy. If you refer to the latter than you should take a look at the programs mentioned earlier such as pymol, inkscape and gimp



Not generalizable
This boils down to what @David already said in his comment:


  You are surely aware that Wikipedia's contributors (not users) are
  completely independent and thus use whatever software they choose to
  create molecular images.