The default renderer—GdiRenderer—is a GDI+ based renderer.  It renders 2D ImageGraphics and VectorGraphics in a BasicPresentationImage’s SceneGraph using the painter’s algorithm.  That is, it recursively traverses the SceneGraph and “paints” each Graphic onto a “canvas” in the order of traversal.  This means that it’s possible for Graphics rendered earlier to be obscured by Graphics rendered later.

For information on how to override/replace the default renderer, see Replace the default renderer.