مشکلم با این کد حل شد
MemoryStream stream = new MemoryStream();
Bitmap bmp = new Bitmap(pictureBox1.Image);
pictureBox1.Image.Clone();
bmp.Save(stream, ImageFormat.Jpeg);
string strFileName = string.Empty;
saveFileDialog1.DefaultExt = "jpg";
saveFileDialog1.InitialDirectory = @"e:\";
saveFileDialog1.Filter = @"Image (*.jpg) |*.jpg|All files(*.*) |*.*";
saveFileDialog1.FilterIndex = 1;
saveFileDialog1.Title = "Export Image";
saveFileDialog1.FileName = comboBoxEx5.Text;
saveFileDialog1.OverwritePrompt = true;
if (saveFileDialog1.ShowDialog() == DialogResult.OK)
{
strFileName = saveFileDialog1.FileName;
FileStream fs = new FileStream(strFileName , FileMode.Create, FileAccess.ReadWrite);
byte[] matriz = stream.ToArray();
fs.Write(matriz, 0, matriz.Length);
stream.Close(); fs.Close();
MessageBox.Show("تصویر با موفقیت ذخیره شد", "پیام", MessageBoxButtons.OK, MessageBoxIcon.Information);
}