The gamelist.XML files are where all of your game data is kept so you can just delete everything you don’t want in each of those files or you can also do that manually by editing the games metadata as mentioned above. Generally speaking the images really are just thumbnails so its probably not that beneficial to have images that at multiple megabytes a piece. Id you want to start with higher quality pics you can take those and run them through an image optimiser online so it keeps a decent quality but doesn’t bog down emulation station. If you get too much going on in emulation station it can cause issues and you have to reallocate ram and stuff but if you optimise your images you should be fine.