That means that Pinterest will be primarily relying on recycled, copyrighted images to draw and maintain their users. The overreaching question that needs to be focused on then is whether or not Pinterest is in violation of US Copyright laws by using images created by another person to advertise and/or sell products.
The looming threat that the internet poses for copyright laws has been well reported on thanks to things like the ill-fated SOPA and the shutdown of megaupload, so many of you reading this blog likely have a good idea of what it entails - the using of another author's work without the creator's permission. This is exactly what is happening with Pinterest. When an image is "pinned", Pinterest copies the image to their servers. Most Pinterest users give a quick nod and link to the original creator, but giving credit where credit is due does not mean they have permission. A lot of artists so far have been happy to receive the publicity, but websites that rely on selling use-rights to things like stock images have not shown themselves quite as amicable.
There has also been some muttering about these images being considered thumbnail catalogs, merely a small/minute representation of the work, and because of this distinction they fall under fair use. But what has to be remembered is that entire images, not representations of them, are being copied to Pinterest servers.
So far, Pinterest has been trying to follow the typical routes to protect their business interests. In order to post anything, users have to state that they hold the rights to the content they pin. Most of the time they aren't telling the truth, but it allows Pinterest to shift the responsibility off of themselves and onto the user. Copyright law has a carve-out provision for publishers of third-party content like Google or Yahoo, meaning they cannot be held responsible for every email, image, and piece of content that has been posted. Pinterest is hoping to fall under the same category, though it is not clear they can be considered a 'publisher'.
Pinterest is also trying to nip their copyright woes in the bud by introducing NOPIN, a metatag function that will allow website owners to easily keep Pinterest users from directly linking the material on the site. Pinterest has also made it clear that it will pursue DMCA claims whenever they are filed. But copyright holders, like publishers, cannot feasibly police every single image they release. To make matters trickier, there are very simple ways around these small barriers and it is unlikely Pinterest would ever be able to draw the interest they have thus far if they only allowed unique content.
It does appear, when all is said and done, that Pinterest is knowingly supplying the means to infringe. As scrutiny mounts they will have to strongly show that it is not their intent to induce infringement, which is an actionable matter in court. But as it stands now, Pinterest and its investors are attempting to monetize infringing activities, which paints a big red target on their back. Though they are doing all they can to make it look as though Pinterest is on the side of the copyright holder, it would likely be them, and not the individual user, who would be pursued in court.