heme pictures (my words for lack of anything better) have always been a part of nude male photography.  In its most simple form, a "theme picture" is a man holding a spear.  However, good theme pictures include more than one man.  Two men wrestling was used and overused.  Others included two men swimming, fishing, in a sauna, etc.  The aim of a theme picture is to allow the viewer to feel he has caught two men in some kind of on going action.  This gimmick draws the viewer's interest deeper in the photo by playing on his curiosity and encouraging him to figure out what is going on. 

The picture of two cowboys on the left is a theme picture at its best.  One glance and the viewer's mind begins to think out a plot on what is about to happen between the two men.  When the viewer's mind is drawn into a photo or painting, the photographer or artist has performed his work well.