Get property id in Windows Forms

I have images in Resources and I would like to get their ID. I haven't found the following code from MSDN to be very helpful.

How did he get the numbers 20624 and 20625?

If I have an image with Properties.Resources.Image1

, how can I programmatically get its ID?

private void DemonstratePropertyItem(PaintEventArgs e)
{

    // Create two images.
    Image image1 = Image.FromFile("c:\\FakePhoto1.jpg");
    Image image2 = Image.FromFile("c:\\FakePhoto2.jpg");

    // Get a PropertyItem from image1.
    PropertyItem propItem = image1.GetPropertyItem(20624);

    // Change the ID of the PropertyItem.
    propItem.Id = 20625;

    // Set the PropertyItem for image2.
    image2.SetPropertyItem(propItem);

    // Draw the image.
    e.Graphics.DrawImage(image2, 20.0F, 20.0F);
}