Firstly, background-<colour>-alpha.png is the image that is onscreen when using the skin. The background WITHOUT the 'alpha' in the file name is a solid image that is shown when the blades slide. Because their are items BEHIND the background as it slides, so we cant have a 82% alpha image like normal, otherwise the illusion would be ruined by seeing the stuff behind it.
We use an alpha image at 82% to show the background animations. This means the wallpapers need to be transparent and the backgrounds almost black, with the animation highlights ranging from black to white so they show through.
Now when making a theme PROPERLY. You need to make the ALPHA and the NON ALPHA images from the same photoshop file. If you do not use this method I am about to explain, your themes will NEVER have a chance of being on XBMC Live.
1. - Make the new file in photoshop and immediatly create a layer that is 100% black (#000000). This is your solid layer. This remains under the backgrounds at all times and has ZERO Alpha/Transparency.
2. - Make your Background layers all 82% transparent. AT ALL TIMES even when saving the none alpha'd image, your backgrounds within photoshop will always be 82% transparent. The BLACK bottom layer acts like the 'nothingness' or 'background animation' in XBMC, directly under your background alpha images, so this black layer is essential to making the transition from your alpha background to the none alpha background.
3. - When saving you image, you can first save the NONE ALPHA image by saving it with the BLACK LAYER below it VISIBLE, so you have your background image which is 82% transparent and the black layer below. Now you might want to increase contrast and saturation a little on images, to bring their colour and brightness back a bit, as being 82% transparent over a black layer makes them seem darker. So save this image as the NONE ALPHA image. Now alls you do to make the alpha image, is make the BLACK LAYER invisible and save again this time as the ALPHA background.
What this does as I explained before, is allow the backgrouns to switch textures from ALPHA to the NONE ALPHA seamlessly, allowing the old page blades and info to slide away underneath it.
Hope this helps budding theme makers understand the background usage in MC360.