public static Bitmap Cut(Bitmap b, int StartX, int StartY, int iWidth, int iHeight)
        {
            if (b == null)
            {
                return null;
            }

            int w = b.Width;
            int h = b.Height;

            if (StartX >= w || StartY >= h)
            {
                return null;
            }

            if (StartX + iWidth > w)
            {
                iWidth = w - StartX;
            }

            if (StartY + iHeight > h)
            {
                iHeight = h - StartY;
            }

            try
            {
                Bitmap bmpOut = new Bitmap(iWidth, iHeight, PixelFormat.Format24bppRgb);

                Graphics g = Graphics.FromImage(bmpOut);
                g.DrawImage(b, new Rectangle(0, 0, iWidth, iHeight), new Rectangle(StartX, StartY, iWidth, iHeight), GraphicsUnit.Pixel);
                g.Dispose();

                return bmpOut;
            }
            catch
            {
                return null;
            }

------解决方案--------------------

b是原始1200*1200图像
C# code

            Bitmap bmp = new Bitmap(1100, 1200);
            Graphics g = Graphics.FromImage(bmp);
            g.DrawImage(b, 50, 0, 1100, 1200);