怎么判断两张图片是否完全相同，通过读取图片内容进行对比

public string GetBase64StringByImage(Image img)
        {
            string base64buffer = string.Empty;

            try
            {
                if (img != null)
                {
                    byte[] bytes = PhotoImageInsert(img);
                    base64buffer = Convert.ToBase64String(bytes);
                }
            }
            catch (Exception ex)
            { }
            return base64buffer;
        }

        //将Image转换成流数据，并保存为byte[] 
        public byte[] PhotoImageInsert(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;
        }

if (pictureBox1.Image == null || pictureBox2.Image == null)
            {
                MessageBox.Show("请先选择两张图片！");
                return;
            }
            string str1 = GetBase64StringByImage(pictureBox1.Image),
            str2 = GetBase64StringByImage(pictureBox2.Image);
            MessageBox.Show(str1 == str2 ? "图片完全相同！" : "图片不相同");