Atop means on the top of something. It tells where something is placed or what is above.
You might see it in older books used like "The book is atop the desk" or "a hat atop his head".
Something is behind you if you cannot see it - your back is facing it rather than your front. The horse in the picture is viewed from behind.
Two things are beside each other if they are next to each other, facing the same direction. The two women on the bench are sitting beside each other, or side by side.
