Tammy, here's a fact you can take to the bank. Native Americans could craft an image in stone as well as any other culture in human history/ prehistory. They were no less artists then the artists of any other time and place.
From this, a certain rule of thumb develops. If you have to guess, if you have to ask at all if a stone is a man made image or a natural image, then the odds are overwhelming that the image is in fact natural. You won't have to squint, use certain lighting, and most important, you won't even have to think about it. Any more then you would ask if a statue carved by Michelangelo was a trick of nature.
With these thoughts in mind, here's a little froggy lookin at ya. Now, honestly, do you even have to ask if this is a trick of Mother Nature?