function addImagetoRSS($content) {
	global $post;

	if ( has_post_thumbnail( $post->ID ) ){
		$content = '' . get_the_post_thumbnail( $post->ID, 'thumbnail', array( 'style' => 'float:left; margin: 0 15px 10px 0;' ) ) . '' . $content;
	}
	return $content;
}

add_filter('the_excerpt_rss', 'addImagetoRSS');