It's every geometer's favorite: the hierarchy of quadrilaterals.  I've used the most basic version here for simplicity's sake, as well as the fact that my last teaching post was with 6th graders, who wouldn't need any more information than this anyway.

The idea that some polygons can be classified as several different things is difficult for many students, so this type of image can help many of them.  Anything on the map can be called itself, or anything that you can get to by going up the map.  Often, a student will see a square, and not understand that the square is also a rectangle (or for that matter, it can be classified as anything on this version of the map).  This map also leaves an opportunity to introduce a new classification of quadrilateral, and have students decide where it fits in to the map.