Just do something like this:
Bitmap memoryImage = new Bitmap(Screen.PrimaryScreen.Bounds.Width,Screen.PrimaryScreen.Bounds.Height);
Size s = new Size(memoryImage.Width, memoryImage.Height);
Graphics memoryGraphics = Graphics.FromImage(memoryImage);
memoryGraphics.CopyFromScreen(0,0,0,0,s);
memoryImage.Save(png_path);
p.s. png_path is file to save
