I had drawn a splash screen (32bpp BGRA) with "transparent" background color in VS2013 and put a pictureBox in a form for display. For me a combination of above answers worked:
public Form1()
{
InitializeComponent();
SetStyle(ControlStyles.SupportsTransparentBackColor, true);
this.BackColor = this.pictureBox1.BackColor;
this.TransparencyKey = this.pictureBox1.BackColor;
}
So make sure you use the same BackColor everywhere and set that color as the TransparencyKey.