You would create a file stream, convert your header text to a byte array and write it to the stream. You'd then read the binary data of the Image and write it to the same file. I haven't tested it but something like this should work:
Code:
using (System.IO.FileStream stream = new System.IO.FileStream("file path here"))
{
byte[] header = System.Text.Encoding.ASCII.GetBytes("header text here");
stream.Write(header, 0, header.GetUpperBound(0));
this.pictureBox1.Image.Save(stream, System.Drawing.Imaging.ImageFormat.Bmp);
stream.Close();
}
Edit: It may be a better idea to write some specifically formatted binary data as the header rather than text, but you get the idea.