I'm not 100% sure how it works, but in Edit > Preferences > Tags (1):
Under Artwork storage there is an option to "copy to the same folder as the music file and name as" with a text box where you can type in a file pattern. Whether the option is checked or not, whatever you type in there gets added to the "Album cover search rules" list on the right-hand side.
If you type "*-cover.jpg" into that field, does it work?
Bee excellent to each other...