error CS0234: 命名空间“System.Drawing”中不存在类型或命名空间名称“Image”

c#开发时报错：

error CS0234: 命名空间“System.Drawing”中不存在类型或命名空间名称“Image”

右键项目，有个引用，添加引用，弹出下面的框

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Net.Sockets;
using System.Net;
using System.Text.RegularExpressions;
using System.Security.Cryptography;
using System.IO;
using static System.Net.Mime.MediaTypeNames;
using System.Drawing;
    string buffurq = msg.Replace("_", "/").Replace("-", "+");
                        switch (buffurq.Length % 4)
                        {
                            case 2: buffurq += "=="; break;
                            case 3: buffurq += "="; break;
                        }
                       

                        byte[] bytes = Convert.FromBase64String(buffurq);