使用专用位图时参数无效

Question

我正在尝试使用在方法外部创建的私有位图在方法中创建新的位图，但它抛出的参数无效错误。我是C＃的新手，所以任何人都可以进行扫描，看看我做错了什么？感谢

    private Bitmap picture;
    private Graphics g1;
    private Cursor c1, c2;
    public HSBColor HSBcol = new HSBColor();
    Pen p = new Pen(Color.Red, 2);

    public Form1()
    {
        InitializeComponent();
        init();
        start();
        mandelbrot();

    }

    public void init() // all instances will be prepared
    {                 
        finished = false;
        x1 = this.Location.X;
        y1 = this.Location.Y; 
        xy = (float)x1 / (float)y1;
        picture = new Bitmap(x1, y1);
        g1 = Graphics.FromImage(picture);
        finished = true;
    }