So none of the sources we use are perfect, and I am fairly sure in saying EmuMovies isn't close either. We are working on adding in two more major scrapers in the future, MobyGames and GiantBomb, which will hopefully help cover european releases, japanese releases, DOS systems and MAME systems a lot more. Some of the more obscure platforms may not have any good solution ever.
If you have an EmuMovies Premium account I believe there are more resources available to you, but I still don't think that covers every MAME game. There is a website or two that have good MAME Metadata, but we are unsure if we can utilize them. So in the future if there is a website that will allow us to use their data and has an API we'd be all over that, but for now what you see is what you get.
As for supplanting your own images. Go to your LaunchBox root folder, here is your images folder. Inside that it sorted by System, then inside a system it is sorted by type of image. You can manually supply your images here as long as the name follows the naming convention. You can see images in here should be named the same name as the game. It may work if some images have ( ) or [ ] as these are ignored in LaunchBox. So if you have or find an images pack and the names match the names in LaunchBox, putting them inside should work just fine. Put the images in and restart LB. I am unsure if they can be named after the rom... it might be just the name in LB, but I don't know, I haven't tested that.