1，判斷圖片的寬和高的比例，然後進行縮放，以便於使縮圖不變形。

2，DrawImage方法為縮圖繪製邊框，這樣，瀏覽起來比較美觀。

3，呼叫Save方法將生成的縮圖儲存到指定的目錄下。

    private void MakeThumbnail(string sourcePath, string newPath, int width, int height)
    {
        System.Drawing.Image ig = System.Drawing.Image.FromFile(sourcePath);
        int towidth = width;
        int toheight = height;
        int x = 0;
        int y = 0;
        int ow = ig.Width;
        int oh = ig.Height;
        if ((double)ig.Width / (double)ig.Height > (double)towidth / (double)toheight)
        {
            oh = ig.Height;
            ow = ig.Height * towidth / toheight;
            y = 0;
            x = (ig.Width - ow) / 2;

        }
        else
        {
            ow = ig.Width;
            oh = ig.Width * height / towidth;
            x = 0;
            y = (ig.Height - oh) / 2;
        }
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        g.Clear(System.Drawing.Color.Transparent);
        g.DrawImage(ig, new System.Drawing.Rectangle(0, 0, towidth, toheight), new System.Drawing.Rectangle(x, y, ow, oh), System.Drawing.GraphicsUnit.Pixel);
        try
        {
            bitmap.Save(newPath, System.Drawing.Imaging.ImageFormat.Jpeg);
        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            ig.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }

    }