The best way I have been able to describe pinterest is like a giant pinboard you may have had growing up, where you put up pictures out of magazines or maybe ones that you took.
Here is the TOS for Member Content
Quote: Member Content We may, in our sole discretion, permit Members to post, upload, publish, submit or transmit Member Content. By making available any Member Content through the Site, Application or Services, you hereby grant to Cold Brew Labs a worldwide, irrevocable, perpetual, non-exclusive, transferable, royalty-free license, with the right to sublicense, to use, copy, adapt, modify, distribute, license, sell, transfer, publicly display, publicly perform, transmit, stream, broadcast, access, view, and otherwise exploit such Member Content only on, through or by means of the Site, Application or Services. Cold Brew Labs does not claim any ownership rights in any such Member Content and nothing in these Terms will be deemed to restrict any rights that you may have to use and exploit any such Member Content. You acknowledge and agree that you are solely responsible for all Member Content that you make available through the Site, Application and Services. Accordingly, you represent and warrant that: (i) you either are the sole and exclusive owner of all Member Content that you make available through the Site, Application and Services or you have all rights, licenses, consents and releases that are necessary to grant to Cold Brew Labs the rights in such Member Content, as contemplated under these Terms; and (ii) neither the Member Content nor your posting, uploading, publication, submission or transmittal of the Member Content or Cold Brew Labs’ use of the Member Content (or any portion thereof) on, through or by means of the Site, Application and the Services will infringe, misappropriate or violate a third party’s patent, copyright, trademark, trade secret, moral rights or other proprietary or intellectual property rights, or rights of publicity or privacy, or result in the violation of any applicable law or regulation.
So basically by uploading your photos, you are allowing them to use it as well, royalty free but they will not claim ownership.