There is a great tool for Android that you should try when you are ready to build the icons for your application. It is called the Android Asset Studio. I used it for the first time last week and it saved me a tremendous amount of time. It takes one image from you and generates the four different sizes needed for your drawable resources folders.
The tool is available here: http://android-ui-utils.googlecode.com/hg/asset-studio/dist/index.html. It is best to use it in Chrome.
The Android Asset Studio has several tools in it. I have so far only used the icon generators. The two things I used were: (1) generating the launcher icon; (2) generating the action bar icons for the set of actions. What it does for you is generate the complete set of images for your drawable folders: drawable-hdpi, drawable-mdpi, drawable-ldpi, and drawable-vhdpi. All you have to do is provide the starter image. It’s great.
Here’s a screenshot of the action bar icon generator:
And here’s what the the launcher icon generator looks like: