private void button2_Click(object sender, EventArgs e) 
        { 
             openFileDialog1.Filter = "*jpg|*JPG|*GIF|*.BMP"; 
             if (openFileDialog1.ShowDialog() == DialogResult.OK) 
             { 
                 string fullpath = openFileDialog1.FileName; 
 
                 //图片异步加载完成后的处理事件 
                 pictureBox1.LoadCompleted += new AsyncCompletedEventHandler(pictureBox1_LoadCompleted); 
                 //图片加载时,显示等待光标 
                 pictureBox1.UseWaitCursor = true; 
 
                 //采用异步加载方式 
                 pictureBox1.WaitOnLoad = false; 
                 Bitmap bm = new Bitmap(fullpath); 
                 Point pt = new Point(bm.Size ); 
                 if (pt.X > pictureBox1.Size.Width || pt.Y > pictureBox1.Size.Height) 
                 { 
                     pictureBox1.SizeMode = PictureBoxSizeMode.Zoom; 
                 } 
                 else 
                 { 
                     pictureBox1.SizeMode = PictureBoxSizeMode.CenterImage; 
                 } 
 
                 //开始异步加载,图片的地址,请自行更换 
                 //pictureBox1.LoadAsync("http://www.); 
                pictureBox1.LoadAsync(fullpath ); 
             } 
        } 
        void pictureBox1_LoadCompleted(object sender, AsyncCompletedEventArgs e) 
        { 
            //图片加载完成后,将光标恢复 
            pictureBox1.UseWaitCursor = false; 
        }
呵呵 这个准行