//1) load it
Bitmap bob = new Bitmap(Image.FromFile(filename));
//fileName is a string. it's the location of the image
//2) diddle with it
b.setPixel(x, y, c);
//x and y are co-ords, c is a Color.
//the bitmap object doesn't contain any decent tools for manipulating itself.
//if you want to do higher level ops than this, just tell me - there are ways, complecated ones.
//3) save it
Image i = Image.FromHbitmap(bob.GetHbitmap());
i.Save(filename, ImageFormat.bmp); //or whatever format you want