By default, WordPress RSS feeds (/feed/) omit featured post thumbnail images. Injecting post thumbnails into RSS feeds enhances feed readers and email newsletters.

WordPress functions.php Code Snippet

functions.phpphp
// Inject featured post thumbnail into WordPress RSS feed content
function lynxbee_rss_post_thumbnail($content) {
    global $post;
    if (has_post_thumbnail($post->ID)) {
        $content = '<div>' + get_the_post_thumbnail($post->ID, 'medium') + '</div>' + $content;
    }
    return $content;
}
add_filter('the_excerpt_rss', 'lynxbee_rss_post_thumbnail');
add_filter('the_content_feed', 'lynxbee_rss_post_thumbnail');