Android app script for Android Studio / Android SDK. This script resizes android xhdpi-sized png image assets into the ldpi, mdpi, hdpi, xhdpi folders. Now just maintain one folder of images rather than 4 or 5. Updated to create directories if needed and also to separately convert images in the mipmap folder (used for app icons)

resizeImagesFromXHDPI.sh

#!/bin/bash -e

# Ensure we're running in location of script.
cd "`dirname $0`"

function fail {
	echo "FAILED with $1"
	exit 1
}

function doConversion() {
	local folderName=$1
	echo "Using $folderName..."

	cd "res/$folderName-xhdpi" || fail "No $folderName-xhdpi folder"

	for f in *; do
	  if [[ ${f} == *.png ]];
	    then
	    echo "...$f"
	    if [[ ${f} != *.9.png ]];
	    then

	    	# ensure folders exist
	    	mkdir -p "../$folderName-xhdpi" || fail "can't make folder"
		mkdir -p "../$folderName-hdpi" || fail "can't make folder"
		mkdir -p "../$folderName-mdpi" || fail "can't make folder"
		mkdir -p "../$folderName-ldpi" || fail "can't make folder"

		convert ${f} -resize 75% "../$folderName-hdpi/${f}" || fail "Failed with ../$folderName-hdpi/${f}"
		convert ${f} -resize 50% "../$folderName-mdpi/${f}" || fail "Failed with ../$folderName-mdpi/${f}"
		convert ${f} -resize 37.5% "../$folderName-ldpi/${f}" || fail "Failed with ../$folderName-ldpi/${f}"	        
	    else
	        echo "not resizing ${f} as it's a 9-patch"
	    fi
	  fi
	done

	cd -
}

# Convert graphic images
doConversion "drawable"

# Convert app icon (now goes in the mipmap folder)
doConversion "mipmap"

echo Complete

This script depends on ImageMagick - try this http://cactuslab.com/imagemagick/