C# Image - System.NullReferenceException

Question

I'm currently trying to update a program by having a button (nextImg) that when clicked calls a function that creates two bitmaps, generates a number starting from 0, and increments every time the nextImg button gets clicked. Then the program takes that number(num) and returns a string from a List[num] which has all the Uri sources of my photos. However when I click next image I get a

System.NullReferenceException.

private void Image_Loaded(object sender, RoutedEventArgs e)
    {
        // Create a new BitmapImage.
        BitmapImage b = new BitmapImage();
        BitmapImage b1 = new BitmapImage();
        Image img = new Image();

        string sSrc = numGen();            
        b.BeginInit();
        b.UriSource = new Uri(sSrc);
        b.EndInit();

        b1.BeginInit();
        b1.UriSource = b.UriSource;
        b1.EndInit();

        img = sender as Image;
        img.Source = b;         // Error on this line

        Canvas1.Height = Picturebox1.Height;
        Canvas1.Width = Picturebox1.Width;
        Canvas1.Margin = Picturebox1.Margin;
    }