This.Why I like apples wallpapers (currently using something from web) is because they are of high quality. And anything that isn't the default wallpapers on the plus models looks terrible in landscape when on the home screen. The wallpapers don't rotate correctly and get zoomed instead.
Apple has gone the extra length to make sure that only default wallpapers scale correctly in both portrait and landscape mode on iPads.
If you use even any 100 mega pixel ultra sharp wallpaper downloaded off the net, iOS manages to make it blurry when it is applied as wallpaper.
Apple has won in making me use only their default wallpapers. Even these leaked iOS 11 GM wallpapers, I didn’t bother downloading them because I know they will look turdy when applied manually.