public static void CompressImage(string sourcepath, string destpath, int quality)
{
    var FileName = Path.GetFileName(sourcepath);
    destpath = destpath + "\\" + FileName;
    using (Bitmap bmpl = new Bitmap(sourcepath))
    {
        ImageCodecInfo jpgEncoder = GetEncoder(ImageFormat.Jpeg);
        System.Drawing.Imaging.Encoder QualityEncoder = System.Drawing.Imaging.Encoder.Quality;
        EncoderParameters myEncoderParameters = new EncoderParameters(1);
        EncoderParameter myEncoderParameter = new EncoderParameter(QualityEncoder, quality);
        myEncoderParameters.Param[0] = myEncoderParameter;
        bmpl.Save(destpath, jpgEncoder, myEncoderParameters);
    }
}

private static ImageCodecInfo GetEncoder(ImageFormat format)
{
    ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
    foreach(ImageCodecInfo codec in codecs)
    {
        if(codec.FormatID==format.Guid)
        {
            return codec;
        }
    }
    return null;
}

private void button3_Click(object sender, EventArgs e)
{
    string[] files = Directory.GetFiles(textBox1.Text);
    DialogResult result2 = folderBrowserDialog1.ShowDialog();
    if (result2 == DialogResult.OK)
    {
        foreach (var file in files)
        {
            string ext = Path.GetExtension(file).ToUpper();
            if (ext == ".PNG" || ext == ".JPG")
            {
                CompressImage(file, folderBrowserDialog1.SelectedPath, (int)comboBox1.SelectedItem);
            }
            else
            {
                MessageBox.Show("The selected file: " + textBox1.Text + " does not contain no imege.", "Compress Unsuccessfull!", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                ClassGV.image = 1;
                break;
            }
        }
        if (ClassGV.image != 1)
        {
            MessageBox.Show("Compressed images has been stored to\n" + folderBrowserDialog1.SelectedPath, "Compress Successfull!", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }
    }
}