Note: First forgive my English, because I am Brazilian and I used google translator.
I have some tips and suggestions to maintain better quality and speed in games database.
First, I have collaborated a lot with ThegamesDB site, and one of the suggestions I learned from them was to convert JPEG images to PNG, as well the file maintains the quality, but is much smaller and thus scraping is much faster.
Another thing, I am in favor of keeping images with the best quality possible, so I favor re-created images or who have had treatment.
On the issue of re-created images, I think it is so important (with so much to maintain the originality). I even collaborate with Hyperspin creating and editing cases, and I've had many times to recreate some bad images elements.
Finally I leave suggestions from some sites that can be used to take images in better quality.
https://gbatemp.net/threads/cover-collections-for-emulators-with-cover-support.324714/
This site contains a large collection of covers of various systems, and mostly form recreated or edited. And for this reason it is always good to take a researched other sites about the fidelity of the cases because some of them not so faithfully recreated, but mostly are excellent and faithful.
And for this reason I will indicate a few more sites that can be used to download cover art, or at least serve as a research to compare the fidelity of the site images that I quoted earlier.
www.segaretro.org
www.guardiana.net
www.nintandbox.net
www.thecoverproject.net