Sadly, no. I guess that the backgrounds are handled differently from the ally/enemies. The allies/enemies images are already transparent backgrounds. The level backgrounds, I don't know why, are different. Unfortunately, I think we will need to keep some of the files as the screenshot versions.
It's not such a big deal, since we only have a dozen or so screenshots out of all the backgrounds. It would be nice if Dominodalek or someone else with the Facebook version could get more complete screenshots. I did notice that if there is dialogue at the start of a level, the full art shows for just a flash before the row of allies appears. I can try to get better screenshots if I can do it fast enough!