Function to add featured image to RSS feed

rssfeedimage.php

<?php
add_filter( 'the_content', 'featured_image_in_feed' );
function featured_image_in_feed( $content ) {
    global $post;
    if( is_feed() ) {
        if ( has_post_thumbnail( $post->ID ) ){
            $output = get_the_post_thumbnail( $post->ID, 'medium', array( 'style' => 'float:right; margin:0 0 10px 10px;' ) );
            $content = $output . $content;
        }
    }
    return $content;
}
?>

I would rather use 'the_content_feed' filter, and 'the_excerpt_rss' filter to make it work with excerpts as well. Then there's no need for is_feed() check, because filters are specific for feed. And we need to check if the feed is for comments or for posts. We don't need post's featured image in comments feed.

<?php
add_filter( 'the_content_feed', 'featured_image_in_feed' );
add_filter( 'the_excerpt_rss', 'featured_image_in_feed' );
function featured_image_in_feed( $content ) {
    global $post, $wp_query;
    if( ! $wp_query->is_comment_feed) { 
        if ( has_post_thumbnail( $post->ID ) ){
            $output = get_the_post_thumbnail( $post->ID, 'medium', array( 'style' => 'float:right; margin:0 0 10px 10px;' ) );
            $content = $output . $content;
        }
    }
    return $content;
}
?>