When it comes to the images that display in the Grow widgets, the first thing Grow looks for is OpenGraph image metadata tags, typically denoted as og:image. Usually this is the featured image you set in your SEO or social sharing plugin.
If there are no OpenGraph image tags then we'll look for any Twitter image tags in the metadata next for the image. If that's not found, Grow will look for a thumbnail meta tag.
If none of above are found on the page, then Grow will pick an image out of the post itself.
TL;DR: If you see the wrong image appearing in Recommended Content, your first step would be to check to make sure that you have og:image set for your posts.