WPF - How to get the URI Source of an Image?

Question

I want to get the URI Source of an image that has been created in XAML. The source of the image is a BitmapFrame. I know you can use BitmapImage.UriSource for BitmapImages, but what about BitmapFrames?

To get the BitmapFrame I use:

    private void imgs_Loaded(object sender, MouseEventArgs e)
    {
        Image img = (Image)sender;
        BitmapFrame frame = (BitmapFrame)img.Source;
        //How to get frame's UriSource?
    }