Personally I prefer all album art in uniform presentation, 1,000 x 1,000 jpg, but it is entirely a matter of taste.
In my experience easily the best source of high quality curated images is https://www.albumartexchange.com/covers
You have to register to get covers without watermarks, but it is free. They have nearly 600,000 user provided images now, although there are many duplicates. It is better for rock, pop and jazz than for classical, but there are more classical covers than you might expect too.