http://wordpress.stackexchange.com/a/75512/131

WordPress convert custom meta field value to post thumbnail

// http://wordpress.stackexchange.com/a/75512/131

if (has_post_thumbnail()) 
    //if the post already has a post thumbnail then just use that
    the_post_thumbnail($size = 'post-thumbnail', $attr = '');
else{
    //if not then convert the custom field to the post thumbnail and display that
    $at_url = get_post_meta($post->ID, 'image', true);
    $at_id = get_image_id_by_url($at_url);
    delete_post_meta($post->ID, 'image');
    if ($at_id){
        set_post_thumbnail($post, $at_id);
        the_post_thumbnail($size = 'post-thumbnail', $attr = '');
    }else{
        //else just display a default image or not :)
    }
}