How to Add Image to WordPress RSS-Feed with no Plug-in?

I have searched for adding featuring image to rss feed with no plug-in for WordPress. I have found some examples and applied how it was instructed. But All I have got now is nothing. Still can’t reach the image.

The last and the best (according to me) solution (as told here) I have found is adding the following code to current theme’s functions.php

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;
}
add_filter( 'the_content', 'featured_image_in_feed' );

I have not seen any change. Is there something else to be done?

What I want to do is adding featuring image as another key in the XML. It would be easier to show it for my slider.

Here is a great example. How to display featured post thumbnails in WordPress feeds

paste this code snippet in your theme functions.php file

// display featured post thumbnails in WordPress feeds
function wcs_post_thumbnails_in_feeds( $content ) {
    global $post;
    if( has_post_thumbnail( $post->ID ) ) {
        $content="<p>" . get_the_post_thumbnail( $post->ID ) . '</p>' . $content;
    }
    return $content;
}
add_filter( 'the_excerpt_rss', 'wcs_post_thumbnails_in_feeds' );
add_filter( 'the_content_feed', 'wcs_post_thumbnails_in_feeds' );