I'm going to use different (better) battle backgrounds for gen 6, but I need those white parts so the pokeballs can be put on it nicely. I'll just put the white fog over the new backgrounds. This will be for the webclient too, it's pretty important!
If you want a white fog on the new backgrounds instead of editing it out of this one the best thing to do is put a white brush over the new image and then edit the opacity and keep it as one whole piece. Im willing to do that if you have a copy of the new battle window, would only take a minute or two, though if you do want it editing out of the above pic then can do that as well :D