Convert the image in a PictureBox into a bitmap

Bitmap bitmap = new Bitmap(pictureBox2.Image)

http://msdn.microsoft.com/en-us/library/ts25csc8.aspx

As per my understanding your have not assigned PictureBox's Image property, so that it is returning null on type cast.

PictureBox property automatically convert the Image format and if you see the tooltip on Image property, it will Show System.Drawing.Bitmap. Check your image property is correctly assigned.

Check this, it is working at my side.

private void button1_Click(object sender, EventArgs e)
{
    var bmp = (Bitmap)pictureBox1.Image;
}

private void TestForm12_Load(object sender, EventArgs e)
{
    pictureBox1.Image = Image.FromFile("c:\\url.gif");
}

/// Using BitMap Class

 Bitmap bmp = new Bitmap(pictureBox2.Image);

You can directly cast pictureBox2.Image to Bitmap as you are doing and also using the Bitmap class to convert to Bitmap class object.

Ref: Bitmap Constructor (Image).

You can find more options here with the Bitmap Class

I think you looking for this:

Bitmap bmp = new Bitmap(pictureBox2.Image)