一，图片转base64字符串

        //图片转base64字符串方法
        public static string ImageEncodeToBase64(Image image)
        {
            System.IO.MemoryStream m = new System.IO.MemoryStream();//实例化内存流.因为MemoryStream前面加了System.IO.所以不用在最上方添加引用using System.IO
            image.Save(m, System.Drawing.Imaging.ImageFormat.Gif);//将图片存入内存流
            string base64string = Convert.ToBase64String(m.GetBuffer());//将内存流中的数据转换为base64字符串，Convert方法
            return base64string;//返回字符串
        }

二，base64字符串转图片

        //base64字符串转图片方法
        public static Image Base64DecodeToImage(string base64string)
        {//使用trycatch语句抓取错误
            try
            {
                byte[] bt = Convert.FromBase64String(base64string);//尝试把字符串存入二进制数组，Convert方法
                System.IO.MemoryStream stream = new System.IO.MemoryStream(bt);//将二进制数组存入内存流
                return Image.FromStream(stream);//返回Image类型数据
            }
            catch
            {
                return null;//抓取错误后的处理方法
            }
        }

三，带界面的转换示例程序

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WFA练习1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private Bitmap bit1;//上提示图片（图片1）
        //private Bitmap bit2;//验证码图片（图片2）
        //图片转base64字符串方法
        public static string ImageEncodeToBase64(Image image)
        {
            System.IO.MemoryStream m = new System.IO.MemoryStream();
            image.Save(m, System.Drawing.Imaging.ImageFormat.Gif);
            string base64string = Convert.ToBase64String(m.GetBuffer());
            return base64string;
        }

        //base64字符串转图片方法
        public static Image Base64DecodeToImage(string base64string)
        {
            try
            {
                byte[] bt = Convert.FromBase64String(base64string);
                System.IO.MemoryStream stream = new System.IO.MemoryStream(bt);
                return Image.FromStream(stream);
            }
            catch
            {
                return null;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (this.openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                string path = this.openFileDialog1.FileName;//获得图片路径
                bit1 = new Bitmap(path);
                this.pictureBox1.Image = bit1;
                textBox1.Text = ImageEncodeToBase64(pictureBox1.Image);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (textBox1.Text=="")
            {
                MessageBox.Show("请输入base64字符串");
            }
            else
            {
                pictureBox2.Image = Base64DecodeToImage(textBox1.Text);
            }
        }
    }
}