I downloaded collection of icons from Android Developers center Each icon in this collection is ordered in forlders according to its resolution: drawable-hdpi, drawable-mdpi
Newer versions of Android support vector graphics, which is preferred over PNG icons. Android Studio 2.1.2 (and probably earlier versions) comes with Vector Asset Studio, which will automatically create PNG files for vector graphics that you add.
The Vector Asset Studio supports importing vector icons from the SDK, as well as your own SVG files.
This article describes Vector Asset Studio: https://developer.android.com/studio/write/vector-asset-studio.html
Summary for how to add a vector graphic with PNG files (partially copied from that URL):
app/build/generated/res/pngs/debug/ folder.