Show Pinterest button when you hover images

I’ve seen some blogs in which the images inside the post, do show a little ‘Pin it’ button in the bottom left corner when you hover over the image.

Is some plugin needed for this, or is it just a piece of HTML code that has to be embedded in every image you include in your posts?

Hope somebody can help me integrating this functionality I would really like to have in the images inside my posts.

Thanks and regards,
Miguel.