Update:
I just understood the problem. The thing is, when you manually add an image inside your template (even between the banner_x_url tag) the image will still be there since the current functions only tracks for the URL itself but
not for the image you added in your template.
Perhaps I should add an option for images from the ACP's configuration page. Will that fit your needs ?
If so, some modifications would also need to be done in order to accomplish this.