C#中对图片的操作主要是通过System.Drawing.Image等类进行。

一、将图片转换为字节流

/// <summary>
    /// 图片处理帮助类
    /// </summary>
    public static class PicProcessHelper
    {
        /// <summary>
        /// 将图片转换为指定的字节流
        /// </summary>
        /// <param name="filePath">图片路径</param>
        /// <returns>指定的字节流</returns>
        public static byte[] ConvertToByte(String filePath)
        {
            var m = new System.IO.MemoryStream();
            var bp = new System.Drawing.Bitmap(filePath);
            bp.Save(m, System.Drawing.Imaging.ImageFormat.Jpeg); //将此图像以指定的格式保存到指定的流中。
            byte[] imgByte = m.GetBuffer(); //从内存缓冲区中读取
            return imgByte;
        }
    }

二、将字节流转换回图片

/// <summary>
        /// 根据字节流返回Image类型
        /// </summary>
        /// <param name="streamByte"></param>
        /// <returns></returns>
        public static Image ReturnImage(byte[] streamByte)
        {
            System.IO.MemoryStream ms = new System.IO.MemoryStream(streamByte);
            Image img = Image.FromStream(ms);
            return img;
        }

三、将Image对象转换为字节流

//将Image转换成流数据，并保存为byte[]   
        public static byte[] PhotoImageInsert(System.Drawing.Image imgPhoto)
        {
            MemoryStream mstream = new MemoryStream();
            imgPhoto.Save(mstream, System.Drawing.Imaging.ImageFormat.Bmp);
            byte[] byData = new Byte[mstream.Length];
            mstream.Position = 0;
            mstream.Read(byData, 0, byData.Length); mstream.Close();
            return byData;
        }

四、保存图片

var oldFilename = @"E:\环境部署\图片集\熊猫1.jpg";
var oldImage = System.Drawing.Image.FromFile(oldFilename);
var newFilename = @"E:\我的新熊猫.jpg";
oldImage.Save(newFilename, ImageFormat.Jpeg);

五、生成缩略图

        /// <summary>
        /// 生成图片缩略文件
        /// </summary>
        /// <param name="originalImage">图片源文件</param>
        /// <param name="width">缩略图宽度</param>
        /// <param name="height">缩略图高度</param>
        /// <param name="mode">生成缩略图的方式</param>
        /// <returns>缩率处理后图片文件</returns>
        public static Image MakeThumbnail(Image originalImage, int width, int height, ThumbnailModel mode)
        {
            int towidth = width;
            int toheight = height;
            int x = 0;
            int y = 0;
            int ow = originalImage.Width;
            int oh = originalImage.Height;
            switch (mode)
            {
                case ThumbnailModel.HighWidth: //指定高宽缩放（可能变形）
                    break;
                case ThumbnailModel.Width: //指定宽，高按比例
                    toheight = originalImage.Height * width / originalImage.Width;
                    break;
                case ThumbnailModel.Hight: //指定高，宽按比例
                    towidth = originalImage.Width * height / originalImage.Height;
                    break;
                case ThumbnailModel.Default: //指定高，宽按比例
                    if (ow <= towidth && oh <= toheight)
                    {
                        x = -(towidth - ow) / 2;
                        y = -(toheight - oh) / 2;
                        ow = towidth;
                        oh = toheight;
                    }
                    else
                    {
                        if (ow > oh)//宽大于高
                        {
                            x = 0;
                            y = -(ow - oh) / 2;
                            oh = ow;
                        }
                        else//高大于宽
                        {
                            y = 0;
                            x = -(oh - ow) / 2;
                            ow = oh;
                        }
                    }
                    break;
                case ThumbnailModel.Auto:
                    if (originalImage.Width / originalImage.Height >= width / height)
                    {
                        if (originalImage.Width > width)
                        {
                            towidth = width;
                            toheight = (originalImage.Height * width) / originalImage.Width;
                        }
                        else
                        {
                            towidth = originalImage.Width;
                            toheight = originalImage.Height;
                        }
                    }
                    else
                    {
                        if (originalImage.Height > height)
                        {
                            toheight = height;
                            towidth = (originalImage.Width * height) / originalImage.Height;
                        }
                        else
                        {
                            towidth = originalImage.Width;
                            toheight = originalImage.Height;
                        }
                    }
                    break;
                case ThumbnailModel.Cut: //指定高宽裁减（不变形）
                    if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
                    {
                        oh = originalImage.Height;
                        ow = originalImage.Height * towidth / toheight;
                        y = 0;
                        x = (originalImage.Width - ow) / 2;
                    }
                    else
                    {
                        ow = originalImage.Width;
                        oh = originalImage.Width * height / towidth;
                        x = 0;
                        y = (originalImage.Height - oh) / 2;
                    }
                    break;
                default:
                    break;
            }
            //新建一个bmp图片
            System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
            //新建一个画板
            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
            //设置高质量插值法
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
            //设置高质量,低速度呈现平滑程度
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            //清空画布并以透明背景色填充
            g.Clear(System.Drawing.Color.White);
            //在指定位置并且按指定大小绘制原图片的指定部分
            g.DrawImage(originalImage, new System.Drawing.Rectangle(0, 0, towidth, toheight),
                        new System.Drawing.Rectangle(x, y, ow, oh),
                        System.Drawing.GraphicsUnit.Pixel);
            return bitmap;
        }

 其中缩略图模式定义如下：

/// <summary>
    /// 缩率图处理模式
    /// </summary>
    public enum ThumbnailModel
    {
        /// <summary>
        /// 指定高宽缩放（可能变形）   
        /// </summary>
        HighWidth,
        /// <summary>
        /// 指定宽，高按比例   
        /// </summary>
        Width,
        /// <summary>
        /// 默认  全图不变形   
        /// </summary>
        Default,
        /// <summary>
        /// 指定高，宽按比例
        /// </summary>
        Hight,
        /// <summary>
        /// 指定高宽裁减（不变形）？？指定裁剪区域
        /// </summary>
        Cut,
        /// <summary>
        /// 自动 原始图片按比例缩放
        /// </summary>
        Auto
    }