Well, if you can use this approach you can save add bigger images with alpha channels into your banner (Retina-Versions 2x etc.). And if scaling is okay for you, when optimizing your Master-Banner as one of the bigger formats to fit into the size constrains (that is how I worked) you can then create all the variations ("Abformate") just by scaling the assets down.
Another interesting thing is the following approach to limit the number of requests and maybe even combine the two ideas. Meaning using a sprite sheet and alpha channel sheet, but I haven't done that yet.