Or perhaps I don’t understand the system. I have cover art for many releases listed in MusicBrainz that are currently missing that art. I click on the Cover Art tab and get “We do not currently have any cover art for Best of the West.” So I click on the “Add Cover Art” button and get
Add Cover Art
Position: (no other cover art)
But none of that is interactive. Below it is a box for editing text in the entry, but I don’t see any way to upload graphics. The FAQ section at https://musicbrainz.org/doc/How_to_Add_Cover_Art shows a system for identifying and uploading art, but I’m not seeing that.