Aric Toler, writing at Bellingcat:
The first and most important piece of advice on this topic cannot be stressed enough: Google reverse image search isn’t very good.
As of this guide’s publication date, the undisputed leader of reverse image search is the Russian site Yandex. After Yandex, the runners-up are Microsoft’s Bing and Google. A fourth service that could also be used in investigations is TinEye, but this site specializes in intellectual property violations and looks for exact duplicates of images.