The 'Pin an Image' entry option is available by upgrading to the basic subscription. The entry option will ask you for a URL of where the image can be found (image file URL) and the URL where the image resides in a blog post (website URL).
Pasting in a Pinterest URL (ex. http://www.pinterest.com/pin/67483694389754481/) won't work with this entry option unfortunately. You'll need to find the image on your site as per the instructions above.
When creating an entry through this option, in the first box that asks you to paste the full URL of the image, you'll need to paste an image file there, like a .jpg or a .png file. You can usually find where this file is located by right clicking on the image on your website and click on copy image URL.
In the second box that asks you to paste the full URL of the page where the image can be found, you'll need to paste in the URL where the image resides, like a blog post or a webpage.