private Point _pt;
  private Point _pt2;
   private void picBoxImageProcessing_MouseDown(object sender, MouseEventArgs e)
    {

   if (e.Button == MouseButtons.Left)
   {
       int ix = (int)(e.X / _zoom);
       int iy = (int)(e.Y / _zoom);

       //reset _pt2
       _pt2 = new Point(0, 0);
       _pt = new Point(ix, iy);

       // pictureBox1.Invalidate();
       picBoxImageProcessing.Invalidate();
   }
}

 private void picBoxImageProcessing_MouseUp(object sender, MouseEventArgs e)
 {
    if (e.Button == MouseButtons.Left && _selecting)
    {
       _selecting = false;
    }
 }

  private void picBoxImageProcessing_Paint(object sender, PaintEventArgs e)
  {


     if (_selecting &&_pt.X >= 0 && _pt.Y >= 0 && _pt2.X >= 0 && _pt2.Y >= 0)
        {

            e.Graphics.DrawRectangle(pen, _pt.X * _zoom, _pt.Y * _zoom, 
                      (_pt2.X - _pt.X)          *         _zoom, (_pt2.Y - _pt.Y) * _zoom);
         }
   } 

       private void picBoxImageProcessing_MouseMove(object sender, MouseEventArgs e)
      {


       if (e.Button == MouseButtons.Left)
        {
         _selecting = true;
         int ix = (int)(e.X / _zoom);
         int iy = (int)(e.Y / _zoom);

         _pt2 = new Point(ix, iy);

         //  pictureBox1.Invalidate();
           picBoxImageProcessing.Invalidate();
         }
     }

there is no problem to draw the rectangle by mouse dragging. But if i  want to change the height or width of the rectangle then I have to draw a  new rectangle, that i don't want. I want to change the height and width  of the rectangle by modifying the drawn rectangle  instead of drawing a new rectangle. 

I don't want to know how to crop. I need to draw a resizable rectangle on the image as we can do in photoshop.

So I need a crop selection control like photoshop's crop control.