I'm far from an expert on programming, but creating a script for matching a game name from the Launchbox DB and to artwork with the same name on a website should be fairly easy to automate right? Here are some sites I recommend for quality box art that could help fill in the gaps. No it won't be perfect but a few low-quality duds out of 1,000s of games fixed seems a whole lot better than no artwork for any of them at all. If the process can't be automated, no harm no foul, still some great resources for everyone to have for manually updating the LB DB.
http://www.thecoverproject.net/
http://www.gamefaqs.com/games
http://www.gamestop.com/
http://cdcovers.to/
http://www.theisozone.com/covers/
https://www.amazon.com/ (has old games but is better for newer games, providing HQ covers, ratings, screenshots, and videos)