How to Add Featured Image to RSS Feeds in WordPress - Add to the content body

functions.php

<?php

function pine_post_thumbnails_in_feeds( $content ) {
    global $post;
    
    if( has_post_thumbnail( $post->ID ) ) {
        $content = '<div>' . get_the_post_thumbnail( $post->ID, 'your-image-size' ) . '</div>' . $content;
    }
    
    return $content;
}

add_filter( 'the_excerpt_rss', 'pine_post_thumbnails_in_feeds' );
add_filter( 'the_content_feed', 'pine_post_thumbnails_in_feeds' );