I wanted to make my windows form transparent so removed the borders, controls and everything leaving only the forms box, then I tried to the BackColor and TransparencyKey to
What works for me is using a specific color instead of the real ability of .png to represent transparency.
So, what you can do is take your background image, and paint the transparent area with a specific color (Magenta always seemed appropriate to me...).
Set the image as the Form's BackgrounImage property, and set the color as the Form's TransparencyKey. No need for changes in the Control's style, and no need for BackColor.
I've tryed it right now and it worked for me...