Here’s the markup in your page:
<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:site" content="@wep_uk">
<meta name="twitter:title" content="Theresa May: leader on a (glass) cliff edge">
<meta name="twitter:description" content="Confronting challenges of female leadership means leading for all women, not just a few.">
<meta expr:content='data:blog.postImageUrl' name='twitter:image'/>
<meta name="twitter:card" content="summary">
<meta name="twitter:site" content="@WEP_UK">
<meta name="twitter:title" content="Theresa May: leader on a (glass) cliff edge">
<meta name="twitter:creator" content="@nellybbb">
<meta name="twitter:image:src" content="http://d3n8a8pro7vhmx.cloudfront.net/womensequality/pages/4183/meta_images/medium/hdr_May.jpg?1507128075">
<meta name="twitter:domain" content="www.womensequality.org.uk">
<meta name="twitter:description" content="Confronting challenges of female leadership means leading for all women, not just a few.">
<meta name="twitter:image" content=" https://d3n8a8pro7vhmx.cloudfront.net/womensequality/pages/4183/attachments/original/1507128119/hdr_May.jpg?1507128119">
You have two sets of cards tags. The cards fetcher is choosing the last one it sees (summary).