Here's how I did it.
set color (list 192 234 250 1)
Make sure the turtle shape is square as when I put it in as a circle, the image imported became bent(curvy).
Of course, some people might like the image to be curved!
Also: note that it's possible to CHANGE a PNG image to have transparent parts using image editing software like the GIMP.
Also, you can create your own 3D shapes by combining the 3D shapes available.