The image needs to be HORIZONTAL. The Twitter format optimizes for 400 pixels wide by 200 pixels high. You can go larger, but Twitter is going to then chop off the edges. I've seen authors use a vertically-oriented book cover for their image and the title gets chopped off! That is less than helpful for branding.
Here's an example post I've made with an image. The image is everything above the bottom grey status bar - so the image includes the cover, the five stars, and the review text beneath.
It's important to note that the image takes up a few characters. So you don't get the full 140 characters to work with once you add an image. You are now reduced to 117 characters. But it's WELL worth it. The power of that image gets you vastly more views and clicks.
It doesn't always have to be a book cover or a product image. If you post about meditating, you can use a stock image of a person meditating on a hill.
There are hundreds of stock image sites out there to buy the rights to images from. Just make sure you always have rights to every image you use. If you use an image you "steal" from the web - of a TV character or something similar - you can be sued for it. Copyright laws can involve heavy penalties. And especially if you're using the image to promote a product, you can end up in serious trouble.
A Note about Images
If you post a link from certain sites - like Amazon - you might get an auto-generated image and think you're all set. But that auto-generated image is flaky. Most of the time it will NOT show. So never rely on those. Always post an explicit image that you have optimized. That way you're sure it will always show up with the tweet.
So, in general, definitely seek to use images with your posts. It makes a substantial difference. Make sure those images are horizontally oriented and test that they show up properly even in the timeline view. Make sure you own rights to images you use.
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