In my case my image path had an extra forward slash, which worked fine in-browser, but wasn't picked up by Twitter. It was a one character error that was hard to spot.
E.g. check for this:
And change it to this:
These are all Open Graph protocol tags. The OG protocol is a metadata standard introduced by Facebook, and understood by many other online platforms like Twitter. While they won't help you with standard search engine indexing and ranking, your website will definitely benefit from them, because they tell Facebook and others how to interpret and display your ...