@
wpbpete - I agree with Adoby - no there is no approved way to do that
But I just dropped a jpeg into an author folder and renamed it to folder.jpg and added 3 books for the author, then I did a couple of conversions and a Save to Disk, I did not download metadata because I know there isn't any (my books are non commercial). The folder.jpg's are still there - Check Library flags them as invalid title, if I edited the author name I'd probably lose the folder.jpg, but I could probably live with those 'hiccups'
Create a small test library based on a couple of existing authors, drop a jpeg into each author folder and rename as folder.jpg and do some tests.
Maybe you can 'integrate' your media server with your calibre library via the Author link feature that's available in calibre Manage Authors facility.
BR