These backgrounds will not be for the entire level. They will switch when you touch a super star (retextured axe npc), then revert back to normal when you collect a power star. (Similar to Mario Wonder.)
They all will be using background 40 as a base, so keep that in mind for the dimensions.
The first background I need must have the tall hill thingies in the distance, as well as some bumpers floating in the air.
Here is a screenshot of what the first level's "Star Effect" looks like:
