Hello Sophie,
The reason for this would be because when using summary_large_image, the image you choose must be at least 280 pixels wide and at least 150 pixels high.
From looking at your tags, I can see the image you’re trying to use in twitter:image is only 53 pixels wide and 53 pixels high.
You will need to resize it. I’ve quickly made two versions that you can save and see if they work.
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280x150 IS transparent
( http://dalehay.me/twitdev/sophiestu-1.png )
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280x150 NOT transparent
( http://dalehay.me/twitdev/sophiestu-2.png )
Hopefully that helps with your problem, if not, just let me know. 