主要利用://在指定位置畫圖
            g.DrawImage(image, new System.Drawing.Rectangle(0, 0, bitmap.Width, bitmap.Height),
                new System.Drawing.Rectangle(0, 0, image.Width, image.Height),
                System.Drawing.GraphicsUnit.Pixel);
image:你的原圖,
System.Drawing.Rectangle:位置與長寬,根據(jù)自己需要設置!
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實例如下:         


   public void MakeSmallImg(string filePath, string saveImg)
        {
            //從文件取得圖片對象
            System.Drawing.Image image = System.Drawing.Image.FromFile(filePath, true);

           
            //取得圖片大小
            System.Drawing.Size size = new System.Drawing.Size((int)image.Width, (int)image.Height);
            //新建一個bmp圖片
            System.Drawing.Image bitmap = new System.Drawing.Bitmap(size.Width, size.Height);
            //新建一個畫板
            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
            //設置高質(zhì)量插值法
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.Default;
            //設置高質(zhì)量,低速度呈現(xiàn)平滑程度
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.Default;
            //清空一下畫布
            g.Clear(System.Drawing.Color.White);
            //在指定位置畫圖
            g.DrawImage(image, new System.Drawing.Rectangle(0, 0, bitmap.Width, bitmap.Height),
                new System.Drawing.Rectangle(0, 0, image.Width, image.Height),
                System.Drawing.GraphicsUnit.Pixel);


            ///文字水印
            //System.Drawing.Graphics G=System.Drawing.Graphics.FromImage(bitmap);
            //System.Drawing.Font f=new Font("宋體",10);
            //System.Drawing.Brush b=new SolidBrush(Color.Black);
            //G.DrawString("myohmine",f,b,10,10);
            //G.Dispose();

 
            ///圖片水印
            //System.Drawing.Image copyImage = System.Drawing.Image.FromFile(System.Web.HttpContext.Current.Server.MapPath("pic/1.gif"));
            //Graphics a = Graphics.FromImage(bitmap);
            //a.DrawImage(copyImage, new Rectangle(bitmap.Width-copyImage.Width,bitmap.Height-copyImage.Height,copyImage.Width, copyImage.Height),0,0, copyImage.Width, copyImage.Height, GraphicsUnit.Pixel);

            //copyImage.Dispose();
            //a.Dispose();
            //copyImage.Dispose();


            //保存高清晰度的縮略圖
            //   bitmap.Save(strGoodFile, System.Drawing.Imaging.ImageFormat.Jpeg);
            //   加個a表示是縮略圖
            bitmap.Save(saveImg, System.Drawing.Imaging.ImageFormat.Jpeg);
            g.Dispose();
            image.Dispose();
            bitmap.Dispose();
        }

=====================================================

c#縮小圖片后不清晰，要怎么縮小才能跟原來一樣清晰。我用：System.Drawing.Bitmap縮小的。

正好之前有寫過這個，不過是用的.net自帶的api寫的，如果想要弄的很好的話非常難，有興趣可以看一下插值法，有若干種辦法不一一列舉。
        /// <summary>
        /// 獲取縮小后的圖片
        /// </summary>
        /// <param name="bm">要縮小的圖片</param>
        /// <param name="times">要縮小的倍數(shù)</param>
        /// <returns></returns>
        private Bitmap GetSmall(Bitmap bm, double times)
        {
            int nowWidth = (int)(bm.Width / times);
            int nowHeight = (int)(bm.Height / times);
            Bitmap newbm = new Bitmap(nowWidth, nowHeight);//新建一個放大后大小的圖片

            if (times >= 1 && times <= 1.1)
            {
                newbm = bm;
            }
            else
            {
                Graphics g = Graphics.FromImage(newbm);
                g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                g.SmoothingMode = SmoothingMode.HighQuality;
                g.CompositingQuality = CompositingQuality.HighQuality;
                g.DrawImage(bm, new Rectangle(0, 0, nowWidth, nowHeight), new Rectangle(0, 0, bm.Width, bm.Height), GraphicsUnit.Pixel);
                g.Dispose();
            }
            return newbm;
        }