Is there a suggested or best practices way of getting a default banner and background for a show? It seems that show has a default portrait picture but not a default background and banner.
No, not currently. The main poster image is selected manually, because it's used on the site itself; banners and backgrounds are pretty much only there to accomodate API users. The plan was to return these images sorted by the amount of upvotes they got, but so far they don't really receive enough votes for that to be useful. (Bit of a chicken-egg I must admit)
Is there a way to expose selecting a default one when editing a show so that users can start selecting one? This could then have main set to true in the API response?