Learn to spot fake or stolen pictures:
http://www.eccie.net/showthread.php?t=655767
Bear in mind that since that video was made, Chrome has included Google Image Search by default. Also, a large number of search engines that archive Backpage ads have come out. This means that an image might get a large number of hits from different places based on just a couple of ads. It makes an unknown or unverified girl seem more legitimate. There is also a search engine that adds all Backpage ads to a page that suggests they are affiliated with Law Enforcement.
Image search is just a tool, but it can be a big help if you know what to look fors.