You would want you custom background image to be the exact resolution that you have your UI set at so Kodi doesn’t have to scale it. I think might also have to override the cache settings to allow larger images with an advancedsettings.xml file with something like this…
<advancedsettings>
<imageres>720</imageres>
<fanartres>1080</fanartres>
</advancedsettings>
…and probably set the background again using a different file name so it get cached again at the higher resolution. Be aware this is going to make your cache larger for any new images you bring in which might slow down your UI. I’ve been using using those settings for a while now on my 3b+ with my entire library cached at that higher resolution though, and not had an issue, so it should be fine.