Worth noting that if you use MC34 it now allows the building of higher quality thumbnails for display in MC and clients (and remotes), assuming the thumbnails you have are already of high enough quality.
However, if you want higher quality cover art you're going to have to find and apply it. Simply upscaling images, at least in my own experience, has never really been that good, unless using some sort of elaborate AI upscaling sort of deal.
