This solution only helps if your image cache hasn't been built yet, or the theme in use doesn't use the image cache in the first place. LB/BB uses a cache system that loads your file then downscales it and saves it out to the cache folder, from then on the UI uses the cache image. Unless explicitly stated to do so the UI doesn't ever display the raw image in the UI (at least not for boxes).
In Big Box you can force the image cache process to pre-populate your cache. This process does take a while especially in larger libraries so is typically something you let run over night.