plugin.php

<?php
/*
 * Plugin Name: Foo
 */

add_image_size( 'my-rss-thumbnail', 292, 136, true );
add_action( 'rss2_item', 'my_rss2_item' );
function my_rss2_item() {
	$thumbnail = wp_get_attachment_image_src( get_post_thumbnail_id(), 'my-rss-thumbnail' );
	if ( ! $thumbnail ) return;
	$thumbnail = $thumbnail[0];
	
	printf( "<image><url>%s</url><title>%s</title></image>", esc_url_raw( $thumbnail ), esc_html( get_the_title() ) );
}