How to draw transparent background image with Camlimages and Graphics

Question

Yesterday I included all the modules of Camlimages Package into my project, I successfully draw images in my graphical window, but the png images (with transparent background) are not drawn with transparent background.

I will show you the image I have, and the result on the graphical window as well as the code.

  let img = Png.load "./images/3mushrooms.png" [] in
  Graphic_image.draw_image img 0 0