The header image was created by Richard Sheinaus, Director of Graphic Design at CUNY Central.  It depicts a tiling of the plane known as the Cairo Tessellation, so-called because it appears in the paving of several Cairo streets.  It has many notable properties, not least of which is that it may be our best approximation of a mathematical impossibility: a regular pentagonal tiling (in the case of the Cairo Tessellation, the pentagonal shape is not regular — not all sides and angles are equal — which saves us from a universe-destroying contradiction).

For more information, check out Wikipedia and the somewhat more exhaustive Dave Bailey’s World of Escher-like Tessellations.