Not the most searchable since it’s main use is through it’s api (I use it through retrodeck/ES-DE which is for ROM libraries) but screenscraper.fr contains scans of boxart, title art, videos from the games, manuals that used to come with physical games and a lot more.

If you want retro ROMs, I can’t help you since I don’t know if I can link to those types of sites but ask on !piracy@lemmy.dbzer0.com and check their wiki linked at the top of the community.