Assuming you are using a PictureBox or similar container, then drawing a cross on it is pretty easy:
using (Graphics g = Graphics.FromImage(myPictureBox.Image))
{
using (Pen p = Pens.Red)
{
Point TLHC = new Point(0, 0);
Point BRHC = new Point(myPictureBox.Image.Width, myPictureBox.Image.Height);
g.DrawLine(p, new Point(TLHC.X, TLHC.Y), new Point( BRHC.X, BRHC.Y));
g.DrawLine(p, new Point(TLHC.X, BRHC.Y), new Point(BRHC.X, TLHC.Y));
}
}
Saving it back to the database is the same method you original used to get in into the database in the first place, but with a UPDATE command instead of INSERT.