People will correct me if I’m wrong, but I believe the practice of showing an image in search results and then hiding that image behind a login wall when users click the link would qualify as cloaking [1], a practice Google specifically prohibits [2].
Unfortunately this and similar rules are not well enforced. The only high-profile case I can remember was when they delisted BMW for a short while for blatant keyword stuffing.
Do they? They advertise images in their search results that you aren't allowed to see when you follow the link. It's some of the most paradigmatic black-hat behavior there is.
You can blame Google for that, how on earth is that Pinterest's fault? Google is the one choosing an arbitrary thumbnail out of hundreds of images to represent "pinterest.com/cake-pops". Then Google uses that thumbnail to refer people to Pinterest, but doesn't tell Pinterest what thumbnail they used. Pinterest's hands are tied here. This is Google's fault, 100%.
Well, considering that a user can't view the image, Pinterest is showing images to Google differently than to the average user. What Pinterest does by hiding the images is specifically against Google's policy. There's no question that Pinterest deliberately went out of their way to create the currently existing paradigm, to suggest their hands are tied and they are somehow a passive victim of Google rather than an actively malicious actor is ludicrous.
It may well be the case that this is Google's fault and they are a malicious actor here as well as Pinterest, but Pinterest is blatantly malicious and does not draw within the lines.
You're seeing the same images that Google does, it's just that these are dynamic pages that change over time. Google might only crawler pinterest.com/cute-cupcakes/ every day, for example. Meanwhile, Pinterest refreshes the content of that page every day, but at a different time than Google crawls it. Boom, you click on the image google showed for "/cute-cupcakes/" and what Pinterest was showing is no longer there to be found. Both Google and Pinterest are misbehaving here. Google is using Pinterest to beef up its image search. Pinterest is using Google to get traffic. But Google won't share any of its precious data with other companies unless they pay for it, so Google doesn't pass anything like a hash or X-ORIGINAL-IMAGE-URL in the referrer headers when you click on a link, so Pinterest's hands are tied, sort of.
Anyway plenty of bad behavior to go around. Both Pinterest or Google could probably solve the issue with better engineering. I put a bit more responsibility on Pinterest, but I'm not giving Google a total pass here either.
