Create image with transparent background using GDI+?

I'm trying to create an image with a transparent background to display on a web page.
I've tried several techniques but the background is always black.
How can I create a transparent image and then draw some lines on it ?

Call `Graphics.Clear(Color.Transparent)` to, well, clear the image. Don't forget to create it with a pixel format that has an alpha channel, e.g. `PixelFormat.Format32bppArgb`. Like this:

var image = new Bitmap(135, 135, PixelFormat.Format32bppArgb);
using (var g = Graphics.FromImage(image)) {
g.Clear(Color.Transparent);
g.DrawLine(Pens.Red, 0, 0, 135, 135);
}

Assumes you're `using` `System.Drawing` and `System.Drawing.Imaging`.

Edit: Seems like you don't actually need the `Clear()`. Just creating the image with an alpha channel creates a blank (fully transparent) image.

This might help(something I threw together which sets the background of a Windows form to a transparent image:

private void TestBackGround()
{
// Create a red and black bitmap to demonstrate transparency.
Bitmap tempBMP = new Bitmap(this.Width, this.Height);
Graphics g = Graphics.FromImage(tempBMP);
g.FillEllipse(new SolidBrush(Color.Red), 0, 0, tempBMP.Width, tempBMP.Width);
g.DrawLine(new Pen(Color.Black), 0, 0, tempBMP.Width, tempBMP.Width);
g.DrawLine(new Pen(Color.Black), tempBMP.Width, 0, 0, tempBMP.Width);
g.Dispose();


// Set the transparancy key attributes,at current it is set to the
// color of the pixel in top left corner(0,0)
ImageAttributes attr = new ImageAttributes();
attr.SetColorKey(tempBMP.GetPixel(0, 0), tempBMP.GetPixel(0, 0));

// Draw the image to your output using the transparancy key attributes
Bitmap outputImage = new Bitmap(this.Width,this.Height);
g = Graphics.FromImage(outputImage);
Rectangle destRect = new Rectangle(0, 0, tempBMP.Width, tempBMP.Height);
g.DrawImage(tempBMP, destRect, 0, 0, tempBMP.Width, tempBMP.Height,GraphicsUnit.Pixel, attr);


g.Dispose();
tempBMP.Dispose();
this.BackgroundImage = outputImage;

}