Add an image to your WordPress RSS feed

functions.php

// Add to functions.php and namespace your methods! :)
function cg_rss_post_thumbnail( $content ) {
    global $post;
    
    if ( has_post_thumbnail( absint( $post->ID ) ) ) {
       $content = '<p>' . get_the_post_thumbnail( asbint( $post->ID ) ) . '</p>' . get_the_content();
    }
    
    return $content;
}
add_filter( 'the_excerpt_rss', 'cg_rss_post_thumbnail' );
add_filter( 'the_content_feed', 'cg_rss_post_thumbnail' );