I recently had a customer who wanted site visitors to be able to pin her photos with the alt description rather than the standard blog post description. I searched through quite a few plugins before I found the jQuery Pin It Button For Images plugin. There are lots of other options with this plugin as well so you can customize it just how you want. The Pin It icon appears when you hover over the image and can be customized as well.
See it in action here: marjiekemper.com/