add_post_images_to_rss_feed.php

<?php
function add_post_image_to_rss( $content ) {
    global $post;
    if ( has_post_thumbnail( $post->ID ) ) {
        $content = '<div>' . get_the_post_thumbnail( $post->ID, 'large', array( 'style' => 'margin-bottom: 15px;' ) ) . '</div>' . $content;
    }
    return $content;
}

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