You can force Pinterest (and any other app that shares an image) to use a specific image if you add the following code to the <head> of your web page as specified here: http://ogp.me/
<meta property="og:image" content="http://ia.media-imdb.com/images/example.jpg" />
Otherwise, our code tries to grab whatever image it thinks is best
Once there you will want to add your URL then click "Debug".
Once your information loads you will want to click "Fetch new scrape information"