There’s no way that’s true. I’d be surprised if even 1% of the Google users paid attention to the source site underneath the image.

And you’d think people with a Pinterest account would opt to search on their website/app rather than going through Google (thus making it even more unlikely that the Google user has a Pinterest account).

Pinterest has a huge user base. They don't need to look at the source if they have an account; the link will work fine.

And, no, most people don't bother picking a site before searching. Why would they artificially limit their results? People don't even visit amazon before searching for Amazon products.

Pinterest’s user-base is surely minuscule compared to the amount of users using Google Image Search.

I’m not saying that 100% of all users with a Pinterest account use their website/app, but many of them will of course opt to use Pinterest’s website/app for their image searches, which means an even smaller percentage of the Google Image Search users will have a Pinterest account.

Not just Image Search - Pinterest often dominates Text Search, and people here are "Oh well, they figured out how to deliver what Google wants!"

But not most people doing a Google search. Anyone who is not on Pinterest has its result page spammed with useless links.