Riddle solved. There was a change [1] on Nov 20, 2013:
Import PIL instead of just the Image class since it doesn’t resolve on all platforms.

-import os, imghdr, urllib, urllib2, sys, Image, argparse, zlib, unicodedata, re
+import os, imghdr, urllib, urllib2, sys, PIL, argparse, zlib, unicodedata, re

Change PIL to Image in scraper.py [2] and resize works again.

[1] https://github.com/elpendor/ES-scraper/commit/37c34f32ef522df2902316318ea876c1a3c11798
[2] /home/pi/RetroPie/supplementary/ES-scraper/scraper.py