Your suggestion works for the viewer alone.... but it does not work if you want to have a design overlay the viewer, the map and other tour components.
Make your image how you wish it to appear. Take a screen capture of the tour in build mode. Crop it to the area of the viewer window. Remove the area not required to overlay the viewer window (I.E keep the bit you need) and save as a transparent .gif. Send the background tour image to back/bottom. Now place the frame image over the top of the viewer. Use the X, Y coordinates to place it 100% correctly. This image will be 100% the same size as your viewer window and the overlay image will align perfectly with the tour background image.
That works great for the viewer window, and we've been using that on some of our tours to round the corners of the viewer, but the same principal doesn't work for the map window. Yes, you can put a transparent frame/image over it, and view the map, but you can't interact with it, as the image is over the map. BUT if you take that same frame image, and slice it into a top edge, side edges, and bottom edge, then they will only cover the edges of the map, and the center will be clear to interact with. Works like a charm really. I have found that if the inner edge of the curves are a neutral or dark color, looks a lot better than a light or white color. Now, what would be nice, would be support for .png with transparency as overlay buttons and frames. One can always dream. Hope this helps. BTW, if anyone wants to see this in action, I'll be happy to post a URL.