(Not sure if this is by design or not)

I'm running with banners in XBMC rather than posters. When I do a "Scrape show with Selected Options" when "Art style" set to "banner" and "Check this if you want to overwrite existing image files" MC always shows a poster when I drill down in the TV Shows "Main Browser" area.

Perhaps there is no Season posters in banner format and it defaults to Poster?

I can't check this without knowing what TV Show you are scraping, but the behaviour is to download your preferred selection in your preferred language only if it is available. If it is not then it will download whatever is available.

Check TVDB.com to see if the show actually has banners, or use the poster browser to view available banners.

If it is any consolation I used to use banners, they looked really good on the Project Mayhem III skin, and TVDB had wide banners for most shows including wide season banners.

It seems that wide banners have largely fallen by the way with newer skins preferring normal poster type images, and TVDB seems to have no season banners at all anymore. I have to admit though that the new skins look superb.