I get a ton of these games that the developers never updated cover art for and all of the ugly horizontal cover art drove me crazy. I found an easy solution and just wanted to share it in case anyone else will find it useful.

https://github.com/Jonius7/steam-missing-covers-downloader/releases/tag/2021_03

Just download the exe and run it, all of your old format cover art will be automatically updated to the new format. How it works is it replaces the cover art with the top rated cover art from steamgriddb.com

I had around 200 games that needed to be updated and the whole process took about 30 seconds. I've read that it might give you problems if you do have a truly massive amount that need to be updated but I had a little over 200 and it worked fine for me.

If you do have a problem with any games not updating let me know, there probably isn't custom artwork in the database, I had one game that didn't update but it wasn't from any bundles, all my bundle games updated fine. But yeah, if you get that problem let me know and I can try to make some custom coverart and add it to the database for you when I have some free time.