Sometimes its best to have image assets pre-scaled and waiting in their folders for Android devices to find and grab.
1. Exporting @ 72dpi web res.
2. Usually you will have maybe low, mid, hd and xhd outputs.
ldpi = 0.00625 * 120px -> 0.75
mdpi = 0.00625 * 160px -> 1.0
hdpi = 0.00625 * 240px -> 1.5
xhdpi = 0.00625 * 320px -> 2.0
xxhdpi = 0.00625 * 480px -> 3.0
xxxhdpi = 0.00625 * 640px -> 4.0
For more see Android’s documentation:
http://developer.android.com/guide/practices/screens_support.html#DesigningResources
