How do I delete all image data from winforms picturebox?

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C#

WinForms

graphics

I have a program which allows the user to open an image and measure it by dragging the mouse over it. The image opens up in a picture box when you press a button. The program works after two images are opened, but not after three images are opened. After the third image, the measurements are grossly overestimated. Here is the part of my code which opens the image:

private void openPlan_Click(object sender, EventArgs e)// open plan folder
    {
        pictureBox1.Image = null;

        // open file dialog   
        OpenFileDialog open = new OpenFileDialog();
        open.InitialDirectory = @"C:\Users\Admin\Documents\complete Lumber Estimation Tool\Plans\";
        // image filters  
        if (open.ShowDialog() == DialogResult.OK)
        {

            trackBar1.Value = 4;
            zoom = 1.0F;
            imgOriginal = null;
            // display image in picture box  
            imgOriginal = new Bitmap(open.FileName);

            pictureBox1.Image = imgOriginal;
       }
  }

Is this mainly a problem with how a windows form program stores data? If so, how do I make sure the program isn't using any data or parameters from the previous image?

What I have tried:

I have tried changing picturebox1.Image = null; to pictureBox1.Image?.Dispose(); . I also tried opening image from a stream and releasing the stream variable by applying stream?Dispose();