SoloCodigo

Espero que este codigo te sirva

Point tmp;

private void pictureBox1_MouseMove(object sender, System.Windows.Forms.MouseEventArgs a){
if (e.Button == System.Windows.Forms.MouseButtons.Left)
    if(tmp==null)
           tmp = new Point(e.X, e.Y);
    else
            g.DrawLine(System.Drawing.Pens.Black,tmp,new Point(e.X,e.Y));
else
   tmp=null;
}