How to create 1024×1024 RGB bitmap image of white?

You almost had it:

private Bitmap DrawFilledRectangle(int x, int y)
{
    Bitmap bmp = new Bitmap(x, y);
    using (Graphics graph = Graphics.FromImage(bmp))
    {
        Rectangle ImageSize = new Rectangle(0,0,x,y);
        graph.FillRectangle(Brushes.White, ImageSize);
    }
    return bmp;
}