[DaveBlake]

What can I do if the artist fanart is not found on-line? Can I use, for example, local fanart images stored locally in order to have the right artist fanart?
As per extrafanart images, I supposed that adding art images for those artists into the same path for extrafanart could get working, but it didn't!

For standard Kodi v17 and below sorry no. Local artwork only works for album artists that have all their music below one unique folder where the local art is placed.

However imporvements are commng in v18, including support for local art and nfo files for all artists. It is available in the current nightly if you are willing to experiment and explore that (with all the usual warnings about using pre-alpha software).

Otherwise some skins and addons do support local art for song artists. Not something I use persoanally, but checking out skin helper service (and the skins that use it) or cdArt addons may help you.