Featured Image by Custom Field and Category

function Function_img_cat($post_id){
    global $post;
    $theme_url = "http://www.redminka.com/wp-content/themes/redminkaepik";
    $img_featured = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'medium');
    if ( in_category('notas') ) {
        $url_imagen = $theme_url.'/img/nota100.png';
    }else   if  ( in_category('enlaces') ) {
        $enlaces_link = get_post_meta( $post->ID, 'url_link', true );
        $enlaces_link_corregido = addhttp($enlaces_link);
        $url_imagen = 'http://s.wordpress.com/mshots/v1/'.$enlaces_link.'?w=400';
    }else   if ( in_category('fotos') ) {
        $url_imagen = $img_featured;
    }else   if ( in_category('videos') ) {
        $video_embed =  get_post_meta($post->ID, 'url_video', true);
        $url_imagen = 'http://img.youtube.com/vi/' .substr($video_embed, -11). '/0.jpg"';
    }else   if ( in_category('documentos') ) {
        $url_imagen = $theme_url.'/img/documento-descargar-100v2.png';
    }else   if ( in_category('articulos') ) {
        $url_imagen = $img_featured;
    }else {}

update_post_meta($post->ID, 'img_categoria', $url_imagen);
}