Add ‘Pin It’ buttons to images when hovered - Wordpress plugins
https://wordpress.org/plugins/jquery-pin-it-button-for-images/ is a free plugin with good reviews, you can control which images can be pinned, and you can use custom images for buttons. See a tutorial about it here: http://www.designisyay.com/free-pin-it-buttons-tutorial/
Update: the developer has created a new plugin with more functionality, also adds sharing to Twitter and Facebook, it’s called Frizzly - https://wordpress.org/plugins/frizzly/
https://wordpress.org/plugins/pinterest-pin-it-button-for-images/ is free but some users report problems with formatting.
http://pinplugins.com/ is a premium plugin with various options, including the hover function. They also sell Product Rich Pins for Woocommerce.
http://pinbuttonattraction.com/?aff=19432 is a premium plugin - hover is the main function.
https://wordpress.org/plugins/cb-pinterest-image-pinner/ is a small, free plugin that adds a Pin It button to all images, some users report issues.