重设图片大小（图片分辨率）

 public Bitmap ResizeImage(Bitmap bmp, int newW, int newH,int dpi)
        {
            var b = new Bitmap(newW, newH);
            b.SetResolution(dpi,dpi);//设置DPI

            var g = Graphics.FromImage(b);

            g.InterpolationMode = InterpolationMode.HighQualityBicubic;
            g.CompositingQuality = CompositingQuality.HighQuality;
            g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.ClearTypeGridFit;
            g.DrawImage(bmp, new Rectangle(0, 0, newW, newH), new Rectangle(0, 0, bmp.Width, bmp.Height), GraphicsUnit.Pixel);

            g.Dispose();

            return b;
        }