难以理解UI Graphics android

At the moment I have just targeted my UI graphics for the hdpi resolutions for android and this works flawlessly, the graphics are designed with 3 resolutions in mind with only the background image being altered really. Problem is the plan was to use the same images for hdpi (480x800 and 480x854) and as well as mdpi (320x480). But when I test it for mdpi with the images in both the -hdpi and copied into -mdpi it doesn't scale them down, do I have to resize all the images and have them duplicated for the mdpi screens?At the moment I have just targeted my UI graphi