<?php
$array = array("12345" => "guarana.jpg");

    $titulo = "Aula 17 - Download de arquivo";

    $idArquivo = $_GET['id'];
header("Content-type: image/jpeg");
    header("Content-Disposition: attachment; filename=refri.jpg");

//the image is in the folder arquivos/guarana.jpg
        readfile("../arquivos/".$array[$idArquivo]);