Yes it is a little strange why it is difficult to change UI’s. Think it might be a bug, but once you do THIS it works as normal afterwards.
Once you select either classic or contempory the background image seems to be random now, as Ive had different background images with different installs.
So whether the volumio team are planning on removing the contempory and classic UIs altogether the /data/backgrounds directory should remain.
There is no harm in creating a directory purely for Now Playing, but it could be a good idea to have an in app way of uploading images to help people who are not familiar with using tools such as sftp.