The digital cover
https://vgmdb.net/db/covers.php?do=view&cover=642859 isn't the same as the CD's cover. (The CD's cover says "TVアニメ「しかのこのこのここしたんたん」テーマソングCD" at the top left, which was entirely deleted without a replacement for the digital release.)
The best quality image I've found is Amazon.co.jp's, but it's not great. Other sites like A-on STORE, toranoana, Rakuten... are both letterboxed and low-res.