If you need the image URLs for the loading screen, they don’t need to be preloaded as they don’t need to be converted to textures, elements etc for HTML.
If you must use resource.getFileUrl(), you can get those when the app fires it’s preload start event as shown here: https://playcanvas.com/editor/scene/783637
Otherwise, it is recommended to use externally hosted images/direct URLS for the preload screen so you don’t have to wait for the asset system to initialise.