This artifact was a simple one made by scaling the original spheres image and adding environment mapping to it.
The extra credit presented is anti-aliasing, environment mapping, and overlapping objects.  To properly load the environment map, go to File->"Load Background Textures..."->"Load *** Texture" (the *** coresponds to Front, Back, Left, Right, Top, and Bottom).  The BMP files that you are supposed to load are located in spheres_scenes.zip and are named appropriately so that you know which ones to load.