I'm not sure about saving an image drawn using e.Graphics as I'm not sure how you could get hold of the object it is drawn to. There is a simpler solution, in the paint method just do somthing like this.
Bitmap myBitmap = new Bitmap(800, 600);
Graphics g = Graphics.FromImage(myBitmap);
g.FillRectangle(mybrush, x * 16, y * 16, 16, 16);
g.Dispose;
this.Image = myBitmap;