This plugin does one thing, and that thing is to add your posts’ featured image to the RSS feed. It specifies the ‘large’ image size.
Many modern feed readers do not show the images added by this plugin. Feedly, for example, will not show images added by this plugin. Feed readers crawl article pages to find the image attached to posts in the open graph meta tag. Many SEO plugins add open graph tags for featured images. I like the (https://wordpress.org/plugins/opengraph/)[Open Graph] plugin, too.