To anyone having this issue, I thought I’d post back my fix after doing lots of research!
It turns out my server didn’t have the GD library installed (discovered after creating a phpinfo.php file). GD library is responsible for all image manipulation within WordPress – resizing and cropping for example. I contacted my host who installed the library. It’s not too difficult to do it yourself but if you’re running WHM on your server, you’ll need to rebuild PHP and Apache, which I was more comfortable leaving to my host (there’s several of our websites on this server).
Hope that helps someone!
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