Question Transparant Form + Transparant PictureBox (PNG)

[Deathspike]

I have a transparent form by setting the background colour to Lime and setting the transparency key to Lime. Now I'm drawing a PictureBox on top of it with a PNG image containing transparent pieces. It's basically a circle with anything besides it transparent. Now, when rendering, somehow the border of the circle will show the Lime color around the edges. Maybe one or two pixels. I really want to get rid of this effect, please help!