Image on top of transparent form

I use the following image in a PictureBox on a simple Windows Form:

http://www.iconarchive.com/show/refresh-cl-icons-by-tpdkdesign.net/Network-Remote-Connexion-icon.html

I want to give the impression that the image is floating on its own, so I have a borderless form, with no caption, and the back colour set to black, which is the same colour as the wallpaper. This works perfectly.

Now I need to use the application on desktops that won't necessarily have a black background.

I thought setting the TransparencyKey to the same value as the BackColor would do the trick, but no matter what colour I use, I get a glow around the image. I have also tried different images with the same results.

Any suggestions?