You should look at libgdx ResolutionFileResolver and also use AssetManager (this will make it easy for you).
, libgdx .
Resolution[] resolutions = { new Resolution(320, 480, ".320480"),
new Resolution(480, 800, ".480800"),
new Resolution(480, 856, ".480854") };
ResolutionFileResolver resolver = new ResolutionFileResolver(new InternalFileHandleResolver(), resolutions);
manager = new AssetManager();
libgdx, res.