Twitter Timeline Content Thumbnail Preview

Since the Twitter widget is added via an iFrame, I’m afraid you won’t be able to customize its output with CSS; we just don’t have control over the content added by Twitter there.

However, you could work around that problem by adding a Featured Image to each one your posts, thus giving an image to each tweet.

Another alternative would be to suggest to Twitter that they add a new option to the Widget configuration screen, allowing you to remove Twitter Cards from each tweet:
https://twittercommunity.com/c/websites

It seems that others already made similar suggestions, so you could add your vote there:
https://twittercommunity.com/t/auto-expand-photos-always-on-for-embedded-timeline/62510

Thanks for the response. Unfortunately (or fortunately) not all tweets are post driven.