Where would I go about looking for banner images in these relative form factors for visual novels in general? My lutris screen looks pretty ugly right now and I'd like to pretty it up some.
In Lutris the banner uses the resolution 184x69, cover art uses 264x352, and icon uses 128x128. Lutris usually grabs the icon from the exe when selecting it, but sometimes it doesn't so in those cases you can use something like icoextract to extract an executable's icon.
All games on Steam have a banner and cover art, so if the VN is on steam you can get the banner with the exact resolution Lutris needs from https://cdn.akamai.steamstatic.com/steam/apps/(app id)/capsule_184x69.jpg (the (app id) part should be replaced with the game's Steam App ID). You can get cover art for a game from https://cdn.akamai.steamstatic.com/steam/apps/(app id)/hero_capsule.jpg, but it won't be the same resolution or aspect ratio that Lutris uses and will need to be cropped.
That works for Steam, but most visual novels don't have a banner with the aspect ratio Lutris expects, so for the majority of VNs you will need to learn how to create something decent with GIMP or any other photo editor.
If I wanted to detail the process of creating a banner from scratch I'd probably need to make a long video about how I usually do it, so it's not something I can describe in text. But basically I usually visit the VN's official site, explore every single page, see if I can find any image that matches something closely to what I want, use inspect element to grab individual image assets and then create something with them that somewhat matches the look of the game's cover art but in banner form.
If I'm lucky and find enough clean assets with transparent backgrounds it might only take like 10 minutes, if I'm less likely it can take a lot longer. Cover art is a lot easier since most VNs have vertical cover art that usually only needs to be cropped a bit, and sometimes need to cut out text/logos and move them a bit if they're too close to the sides that need cropping. If the VN's cover art is horizontal, then the process is a lot closer to when creating banners but not nearly as difficult and time consuming.
While in the end this is rather pointless and not something most people would ever want to do, at least it makes my Lutris library a lot nicer to look at.