By default, Android scales your bitmap drawables (.png, .jpg, and .gif files) and Nine-Patch drawables (.9.png files) so that they render at the appropriate physical size on each device.
http://developer.android.com/guide/practices/screens_support.html
But you did not wast effort and time because to ensure your bitmaps look their best, you should include alternative versions at different resolutions for different screen densities. Also this will give better performance