How to deal with icons and images at different resolutions?

I just realized I don't really know what I'm doing again. My first app is coming along nicely within the emulator at 320 by 480 at 160dpi. But I don't know what to do with images and icons when moving to different resolutions. Right now I have some images assigned to buttons with StatelistDrawable. But will those buttons scale larger automatically on a larger device or do I do this in the designer? And if they do, I imagine the images will be streched and not look very good. Should I just start with larger images and set them to fill and scale down on a smaller screen, or have different sizes of each image in assets and load according to the screen size? Thanks.