Here a quick plugin for all the Pinterest addicted: Pinterest for Galleries. To be precise “for WordPress galleries used on posts”.
Pinterest for Galleries simply adds the “pin-it” button under each thumbnail of a WordPress gallery letting readers to “pin” exactly that image, not the full post!
This is the way Pinterest should be used, right?
The plugin may not work on some themes or may not render well, so install, activate, try it and, eventually, deactivate it. There is no options panel at this development stage.
A demo gallery
Unser each thumbnail of this gallery you should be able to see the “pin-it” button.