Oh you didn't bait the know it all uncle, the know it all uncle has baited you!
Oh right. You may have noticed the cover page pic for the manga has changed (used to be vol 17's cover). I don't know if this is automatic or if someone with admin permissions for the manga did it. I actually can't do anything to the main page like add pics or change the description. I believe I need to be a power uploader with 500 ups (soon) or a "group leader".
Actually anyone with an account can "make" changes to a mangadex entry, or more accurately request changes.
This can be done by clicking on the report button of an entry, the flag that is next to the upload button below the tittle of an entry.
Then an admin will make the change, this way you can change all pertinent aspects of an entry's information, including volume covers.
And i think, but don't quote me on this specific part, but an entry will always display the latest volume cover available for its main cover
Volume covers can be reported by anyone as long as you can show to the admins the existence of a cover, and then they will add them to the information of an entry under the art tab, you can even add back covers, inner covers, and alt covers this way, by for example naming them things like volume 11.1 or 11.2, 11.b, etc
So yeah, you don't need to become a power uploader, or a group leader, or any of that fancy stuff, any normal chump can do it