For some, this isn't a concern; it all depends on why and how you're using Pinterest. For others, there's a good reason to be concerned. For example, if I were to design a logo or a mockup of a website for a client and then upload the images to Pinterest for my client's viewing, upon uploading those images I have granted Pinterest "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."
Terms of Service are generally written with the best interest of the entity that generated those Terms, as opposed to the individuals agreeing to abide by them. Before you click the "I Agree" box, always be certain that you know what you're agreeing to.
Worried about Pinners pinning images from your website? In the interest of avoiding copyright infringement issues, Pinterest has introduced a simple metatag to be added to the head of websites that will disable Pinterest's "pinning ability" on that particular website: