I learned the hard way how important thumbnails are for a post, even if a thumbnail is mostly text with minimal to no graphics. Here a consistent look and feel are a must.
Once I started using thumbnails, I noticed it wasn't enough: they needed to be the right dimensions. Not knowing what you had explained in this post, I aped into 600x340 dimensions after measuring the thumbnail as shown in feeds. In the feeds they measure 150x85, so I just multiplied each dimension by 4.
It turns out that the dimensions I used are in proportion to the 16:9 you not only recommend but urge everyone to use.
I don't have a Twitter account, so I never considered image sizes used there. I'll need to account for that going forward.
New for me would be the importance of a buffer zone around the image core to account for image cropping done by various platforms. That's good to know.
It's too late for me to upvote this post, so please accept this slice of !PIZZA instead.