In http://test.musicbrainz.org/release/5e0fa610-ec47-4c2d-a92a-08cf842b76e2/cover-art front = booklet 1, so I asked it to order it as Front Booklet(s) Medium Back - which is to me the reasonable and expected order. It didn't say anything about it, until I applied the edit and the back stayed where it was (all else moved). If we're going to keep /back as second image by design, we should say so when reordering. In any case, it makes for very confusing UI - I'd MUCH rather show /front and /back on their own at the top, then again in a list of all cover art in the place selected by editors.