winform在上传图片时改变图片大小

//打开选择文件对话框
OpenFileDialog OFG = new OpenFileDialog();
OFG.ShowDialog();
this.pictureBox1.Image = Image.FromFile(OFG.FileName);
Image img = new Bitmap(pictureBox1.Image);
　　　　　　　//在这设置你想要的Size  目前设置为 宽800 高1100
Image changeSize = img.GetThumbnailImage(800, 1100, null, new IntPtr());
pictureBox1.Image = changeSize;
label5.Text = "图片实际像素值:" + pictureBox1.Image.Width + "×" + pictureBox1.Image.Height;