The photograph shows the side and top-bottom view of the large plates on the back of the Stegosaurus.  Originally thought to have been a form of armour-plating, the plates of advanced stegosaurs were, as can be seen here, actually quite thin and full of holes for the accomodation of blood vessels.  For all large animals heat disipation is a big problem (that is why African elephants have such large ears for example), and the Stegosaur plates clearly served this purpose.  The plates tend to be small and poorly developed (or even absent) in primitive Stegosaurids, but more developed in more advanced forms, indicating that they conferred an adaptive advantage.