<?php

function ccd_rss_featured_images( $content ) {
    
	global $post;
    
    // For versatility, apply a filter to the size returned.
	$size = apply_filters( 'ccd_rss_featured_image_size', 'medium' );
    
	if ( has_post_thumbnail( $post->ID ) ) {
		$content = sprintf( '<div>%1s</div><div>%2s</div>', get_the_post_thumbnail( $post->ID, $size, [] ), $content );
	}
    
	return $content;
}
add_filter( 'the_excerpt_rss', 'ccd_rss_featured_images' );
add_filter( 'the_content_feed', 'ccd_rss_featured_images' );

?>