We must be able to combine ResourceLoaderImageModule's icon CSS and regular CSS files in one module. Otherwise we can't use ResourceLoaderImageModule to ship OOjs UI's "core" icons, indicators and textures (which have to be included in the 'oojs-ui.styles' module).
Since we can't do dependencies for CSS-only modules (T63577), the only way is probably to make ResourceLoaderImageModule inherit from ResourceLoaderFileModule.