Hm, I tried making copies of beach.webp and renaming them for my 'custom' locations, and that didn't seem to work either.
My next thought just now is that the game might not be able to parse location names that have a space in them (e.g. 'Skating Rink' instead of 'SkatingRink') so I tried changing that and will test again soon.
Thanks for following up by the way :)