Add these line of codes to functions.php of your theme; to display thumbnail of featured image of the post. #WordPress

functions.php

/*===================
    ADD IMAGE TO FEED
  ===================*/

// display featured post thumbnails in RSS feeds
function WPGood_rss_thumbs( $content ) {
    global $post;
    if( has_post_thumbnail( $post->ID ) ) {
        $content = '<figure>' . get_the_post_thumbnail( $post->ID, 'large' ) . '</figure>' . $content;
    }
    return $content;
}
add_filter( 'the_excerpt_rss', 'WPGood_rss_thumbs' );
add_filter( 'the_content_feed', 'WPGood_rss_thumbs' );
/*===================
    ADD IMAGE TO FEED ENDS 
  ===================*/