float actualHeight = myImg.Height;
 float actualWidth = myImg.Width;
 float imgRatio = actualWidth / actualHeight;
 float maxRatio = (float)this.Width / this.Height;

                if(imgRatio!=maxRatio)
                {
                    if (imgRatio < maxRatio)
                    {
                        imgRatio = this.Height / actualHeight;
                        actualWidth = imgRatio * actualWidth;
                        actualHeight = this.Height;
                    }
                    else
                    {
                        imgRatio = this.Width / actualWidth;
                        actualHeight = imgRatio * actualHeight;
                        actualWidth = this.Width;
                    }
                }
 pictureBox.Size=new Size((int)actualWidth,(int)actualHeight);
 pictureBox.Location = new Point((int)((this.Width - actualWidth) / 2), (int)((this.Height - actualHeight) / 2));