Just like Facebook and Twitter, Pinterest has its own namespace. Go like this:
<meta property="pin:url" content="https://yourpage.com/pinit/test/pinterest_meta.html" /> <meta property="pin:description" content="This pin was generated by the Pinterest meta." /> <meta property="pin:media" content="http://cdn.yourpage.com/your_image_here.jpg" />
... and your preferred image (which need not actually appear on the page) will show at the top of the grid when someone uses Pinterest's mobile device, browser extension, or Any Image Save button.