I had a look at your site, and it seems that most of your latests post do not include any image. As a result, Jetpack doesn't include any Image Open Graph Meta tag to your site's
Open graph meta tags are used by Facebook to gather information about your site. For example, in your latest post, you can find the following meta tags:
<meta property="og:type" content="article" />
<meta property="og:title" content="The most important document EVER for year 11 GCSE geography students " />
<meta property="og:url" content="http://blog.wintoncollege.org.uk/2013/04/the-most-important-document-ever-for-year-11-gcse-geography-students/" />
<meta property="og:description" content="The case study booklet, includes ALL the case studies you need to know for your terminal exam. The three topics you will be examined on are; Natural Hazards, Rivers & Coasts and finally Economi..." />
<meta property="og:site_name" content="Winton College" />
If Facebook doesn't find an image meta tag, it will crawl the page and look for an image: if it finds an image that's larger than 200px x 200px, it will be used on Facebook. If all images are smaller than the required dimensions, you will experience issues.
The best way to solve the problem would be to insert an image into your post. Another solution would be to use a plugin that will add an Open Graph image tag to your posts, even when you choose not to insert any image into the post.
You could use OpenGraph, but you can also look for other similar plugins in the plugin repository.
I hope this helps.