Android devices come in a great deal of different flavors. You have slow devices, fast devices, crappy devices, great devices, lowres devices and ultra-hd devices.
When you want your app to look great in various screen densities, the app’s image assets should be exported for the following densities:
These densities use a 2:3:4:6 scaling ratio, which means that
- 1 MDPI = 1 MDPI
- 1 HDPI = 1.5 MDPI
- 1 XHDPI = 2 MDPI
- 1 XXHDPI = 3 MDPI
For instance, the launcher icon is 48, 72, 96 or 144 pixels wide (mdpi, hdpi, xhdpi, and xxhdpi), depending on the screen density.
If you look at the Android Developer Iconography Page, it suggests that you should create your launcher icon in 864x864 pixels. This will provide you with an image that can be evenly divided to the various sizes.
This means that when you design artwork for an app, you should either use vector based assets that can be exported in any resolution, or create original graphics to be 18 times larger than the MDPI pixel size (864/18 = 48). This results in an image asset that can be evenly divided for all available screen densities.