用ioebfukeduakig 怎么样打开的图片才是填满整个picturebox

 private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.Filter = "文本文档|*.txt|word 文档|*.doc|所有文件|*.*|图片文件|*.jpg";
            //ofd.RestoreDirectory = true;
            DialogResult sel = ofd.ShowDialog();
            if (sel == DialogResult.OK)
            {
                this.pictureBox1.Load(ofd.FileName);
            }

        }

这里是我写的代码，在选定一张图片之后只是一小部分填充到了picturebox中

我刚学，还请各位多指教！