Show image with highest rated post

Sorry you can’t, the code outside the loop doesn’t doesn’t parse any custom field in the template.

You can however use http://codex.ww.wp.xz.cn/Function_Reference/query_posts <?php query_posts( array( 'meta_key' => 'ratings_average', 'orderby' => 'meta_value_num', 'order' => 'DESC' ) ); ?> to get the highest rated post and you can style it using any normal template tags.