HubSpot's COS Blog featured image tag allows users to specify an image from within a blog post (anywhere in the post) that is used as the thumbnail for sharing the post over social media channels and can also be used as the thumbnail on the blog listing summary page, if the option has been enabled.
To enable the featured image option for your COS Blog listing page please navigate to Content > Content Settings > Blog.
Under General, check off the Use featured image in listing summaries option, followed by clicking on Save changes.
Lastly, within the Settings tab of your blog post, ensure the "Featured Image" option is also Enabled (this option will be turned on by default.) You can choose any one of the images available here as your featured image, or you can even click on the [ + ] icon to add/upload a whole new image that wasn't used in the blog post.
With that said, please be aware of the following scenarios pertaining to this feature:
1. If you have an image in your blog post before the Read More tag, and select the featured image setting as just instructed, we'll assign such image as the post's featured image.
2. If an image is after the Read More tag, within the body of the post, it will be moved to the upper right of the blog listing summary text which appears on your Blog Listing Page, as the post's featured image (as long as the Featured Image checkbox is selected).