How to overlay an image over the entire slideshow

I'm glad that you like the module. :)

There are two ways of overlaying something over each slide: changing the slide images themselves or presenting a transparent image over-top of the existing slide images.

The first way involves taking advantage of Nivo Slider's image style integration by using the ImageCache Actions module to "bake" an overlay into each slide image.

The second way requires you to manipulate the "ribbon" <div> above the slideshow. You can combine a transparent background with absolute positioning and a fixed height + width to overlay the background over the slideshow. This is how the default Orman and Pascal themes add a ribbon to the corner of the slideshow (you can take a look at their CSS for examples).

You'll want to do something like:

.theme-themename .ribbon {
    background: url(background.png) no-repeat;
    width: 100px;
    height: 100px;
    position: absolute;
    top: -50px;
    left: 50px;
    z-index: 300;
}

Set the height and width to that of your image dimensions and position the image using top and left. Make sure that you set the z-index high enough to ensure that the background is rendered before the slides.