位图到base64

Question

如何将位图转换为base64字符串？这是我到目前为止尝试过的：

    Private Function solve_captcha(image As Bitmap) As String
    Dim encode_type As ImageFormat = ImageFormat.Jpeg
    Using ms As New MemoryStream
        image.Save(ms, encode_type)
        Dim imageBytes As Byte() = ms.ToArray
        Dim base64str As String = Convert.ToBase64String(imageBytes)
        Clipboard.SetText(base64str)
        MsgBox(base64str)
    End Using
End Function

但是输出始终是黑色图像...