// THIS INCLUDES THE THUMBNAIL IN OUR RSS FEED
function insertThumbnailRSS($content) {
    global $post;
    if ( has_post_thumbnail( $post->ID ) ){
        $content = '' . get_the_post_thumbnail( $post->ID, 'art-slide-big', array( 'style' => 'float:left; margin-right:10px; height: 150px; width: 300px; border: 10px solid #e5e5e5' ) ) . '' . $content;
    }
    return $content;
}

add_filter('the_excerpt_rss', 'insertThumbnailRSS');
add_filter('the_content_feed', 'insertThumbnailRSS');