Somthing like this could be a good place to start
var fd = new SaveFileDialog();
fd.Filter = "Bmp(*.BMP;)|*.BMP;| Jpg(*Jpg)|*.jpg";
if (fd.ShowDialog() == System.Windows.Forms.DialogResult.OK)
{
switch (Path.GetExtension(fd.FileName))
{
case ".BMP":
pictureBox.Image.Save(fd.FileName, System.Drawing.Imaging.ImageFormat.Bmp);
break;
case ".Jpg":
pictureBox.Image.Save(fd.FileName, System.Drawing.Imaging.ImageFormat.Jpeg);
break;
default:
break;
}
}