What I have been doing is to upload the new cover art I scan, putting it to the front of the list, then marking the replaced art for deletion.
If you are going to remove it, make sure that it is exactly the same cover, back, etc. as the one you are uploading and that yours is substantially better. I missed some small differences on a couple albums - fortunately someone caught me and stopped it. Turns out I actually had a new release in the group!