正如Fildor所说，使用base 64对图像进行编码。
我用winform（. Net Framework4.8）进行了测试，并修改了您的代码。
我自定义了一个txt文件的保存地址和用户名。并使用openfiledialog来选择需要上传的图像地址。
下面是我的演示：

private void button1_Click(object sender, EventArgs e)
{
    var openFileDialog = new OpenFileDialog();
    openFileDialog.Filter = "All Files (*.*)|*.*";
    openFileDialog.Title = "Select a file";
    if (openFileDialog.ShowDialog() == DialogResult.OK)
    {
        string filename = openFileDialog.FileName;
        var stream = new System.IO.MemoryStream();
        using (var customavatar = Image.FromFile(filename))
        {
            customavatar.Save(stream, ImageFormat.Jpeg);
        }
        var base64String = Convert.ToBase64String(stream.ToArray());

        // Create the file, or overwrite if the file exists.
        using (var fstream = new FileStream(textfile, FileMode.Create, FileAccess.Write, FileShare.None))
        {
            using (var writer = new StreamWriter(fstream))
            {
                writer.Write($"{username},{base64String}");
            }
        }
    }
}

private void button2_Click(object sender, EventArgs e)
{
    string[] filelinesarray = File.ReadAllLines(textfile);
    foreach (string str in filelinesarray)
    {
        if (str.Split(',')[0] == username)
        {
            byte[] byteArray = Convert.FromBase64String(str.Split(',')[1]);
            MemoryStream stream = new MemoryStream(byteArray);
            Image img = Image.FromStream(stream);
            pictureBox1.Image = img;
        }
    }
}