I am not an attorney and any comments I post are not intended, nor should they be construed, as legal advice. If you need legal advice, please consult a legal expert who is familiar with the area of legal expertise you need.
I recently added the Pinterest extension to Firefox. As a result, I now see the Pinterest Save button on images on sites like Find-A-Grave, Kickstarter, and numerous other websites.
Unless something has changed, Pinterest made it clear that if you pin an image, they will hold you responsible if you violated copyright in pinning an image. That’s true even if you re-pin an image added by another Pinterest member.
I wouldn’t recommend using the Pinterest Save option on anything you post publicly to Pinterest. Some copyright holders will have no problem filing a take-down notice and pursuing legal action against you for violating copyright. The current copyright holder for Lord of the Rings has no problem going after those who violate copyright.