C#

public void ShapeTheForm(Bitmap ShapeImage, Color Key)
{
    if (busy) return;

    busy = true;
    // Instantiate a region to keep the transperant parts of the bitmap
    Region trans = new Region();
    // A region is set to infinate by default, and we want an empty region in order to add the pixels to it
    trans.MakeEmpty();

    int width = ShapeImage.Width;
    int height = ShapeImage.Height;

    for (int y = 0; y < height; ++y)
        for (int x = 0; x < width; ++x)
        {
            // Note: Comparison is not made between two Color objects on purpose.
            //       If a known color was set as the key (e.g. Fuchsia, Magenta...) the key name would be the known color,
            //       but the color returned by GetPixel() would have a hexed name, and the two would mistakenly not be equal.
            Color pixel = ShapeImage.GetPixel(x, y);
            // If the current pixel is supposed to be transparent...
            if (pixel.R == Key.R && pixel.G == Key.G && pixel.B == Key.B)
                // ... add the pixel to the transparent region
                trans.Union(new Rectangle(x, y, 1, 1));
        }

    // Initialize a temp region the size of the image
    Region tempRegion = new Region(new Rectangle(0, 0, ShapeImage.Width, ShapeImage.Height));
    // Exclude the transparent region from the full region
    tempRegion.Exclude(trans);
    // Remember the current border style
    lastStyle = FormBorderStyle;
    // Set to shaped layout
    if (InvokeRequired)
        Invoke(new LayoutSetDelegate(SetShapedLayout), ShapeImage, tempRegion);
    else
    {
        // Remove the form's border
        FormBorderStyle = FormBorderStyle.None;
        // Set the size of the form to the size of the image
        Size = new Size(ShapeImage.Width, ShapeImage.Height);
        // Set the form's region to the temp region
        Region = tempRegion;
        // Set the background of the form to the image
        BackgroundImage = ShapeImage;
    }

    // Release resources
    trans.Dispose();
    trans = null;

    busy = false;
}

Region = tempRegion;