When I use Thumbnail mode for viewing my Music collection I see Album cover instead of plain folder icon.
But when in some folder only 1 (one) music file inside, I see plain folder icon with picture on it.
See attached screenshot and bottom line of folders where six folders contain only one .mp3 inside.
Also, if I will change album cover in WMP, folder.jpg will be changed automatically, but coverart.jpg won't be replaced and dopus will show older image.
Opus does read folder.jpg. coverart.jpg just forces it to use a CD thumbnail instead of auto-detecting whether or not the folder seems to be an album.
You can always create a softlink from coverart.jpg to folder.jpg if you need it so updating one updates the other. (How often do album covers change, though, and how many albums only have one or two tracks?)