Dude, as for images, you really have to generate several different sizes, if you want the app to work well on different devices, using various drawable folders.
But I have two tips for you, which are a hand in the wheel:
1st To build icons for your application ( icon_launcher / icons for actionbar / navigation drawer / or even create an icon with android defaults, from any image ) use this site < a href="http://android-ui-utils.googlecode.com/hg/asset-studio/dist/index.html?utm_source=twitterfeed&utm_medium=twitter"> here and be happy:)
2nd One way (not the most correct, but one that works best) is to create several values folders (examples: values-normal-hdpi / values-normal-mpdi / valuessw600dp ), with a different styles.xml file for each of them.
So you use a single image with a good resolution and change the width and height for each screen resolution.
And you can still use these styles as your own library for use in other applications.
If you want to understand better, just talk:)