Add media and images to rss feed in WordPress.

functions.php

<?php

/**
 * Add 'Featured Image' to a rss feed.
 *
 * @return void
 */
function add_feed_item() {
	$query = get_queried_object();

	// Add featured image to 'Featured' events.
	if (
		'tribe_events' === get_post_type() &&
		is_a( $query, 'WP_Term' ) &&
		'featured' === $query->slug
	) {

		$featured_image = get_post_thumbnail_id( get_the_id() );

		if ( $featured_image ) {
			$image = wp_get_attachment_image_src( $featured_image, 'thumbnail' );
			list($url, $width, $height) = $image;
			$image_path = str_replace( home_url(), ABSPATH, $url );
			$mime = getimagesize( $image_path )['mime'];

			printf(
				'<media:content url="%s" type="%s" height="%s" width="%s" />',
				$url,
				$mime,
				$height,
				$width
			);
		}
	}
}
add_action( 'rss2_item', 'add_feed_item' );

/**
 * Add Media namespace to feed.
 *
 * The media namespace is required to make the <media> tag valid for readers.
 *
 * @return void
 */
function feed_namespace() {
	echo 'xmlns:media="http://search.yahoo.com/mrss/"';
}
add_action( 'rss2_ns', 'feed_namespace' );