How to Add Featured Image to RSS Feeds in WordPress - Add as an individual item

functions.php

 <?php
 
function pine_feed_add_image( ) {
    global $post;
    $output = '';

    if( get_post_type() !== 'post' ) {
        return;
    }

    if( has_post_thumbnail( $post->ID ) ) {
        $output = '<media:content xmlns:media="http://search.yahoo.com/mrss/" url="' . get_the_post_thumbnail_url($post->ID,  'your-image-size' ) . '" medium="image" type="image/jpg" />';
    }

    echo $output;
}

add_filter( 'rss2_item', 'pine_feed_add_image' );