You already know you should have at least one picture for every blogpost for both attention and SEO.
Before Pinterest became popular horizontal images or square ones were the norm, not just for blogs but for social media channels as well.
Most blog themes are styled for horizontal images, and those are not ideal if you want attention in the Pinterest-feed. Even if your theme allows for vertical images you may not want to use them, as they take up a lot of space before the reader even gets to the actual article.
How can you solve this? One way, of course, is to use vertical images for your blog posts, but then be aware of how they look when shared on other social channels but Pinterest.