Tomer, this was a really great read! I’m such a geek about A/B testing pretty much anything and user-generated photos, so this really is the best of both worlds.
Stock photos do fail to tell a story about a brand, which is what UGC truly excels at. I think your experiment shows that perfectly. Though customers might enjoy looking at plain old product shots for detail’s sake, getting inspiration from others — this time in UGC form — is really how we operate. Fashion really exemplifies this, too. I love it when companies show how their customers style certain pieces, à la your Zara example.
I work for Scopio, which is a company that curates UGC for brands. We’ve seen great results even in B2B and nonprofit sectors. One of our most successful and rewarding projects was with the UN: https://www.scop.io/blog/ugc-gallery-unep-scopio.
Let me know what you think!