Put this code in top of your file. and display your image as you are displaying in HTML file. It will download as excel file as you want

$file_type = "vnd.ms-excel";
$file_name= "somename.xls";
header("Content-Type: application/$file_type");
header("Content-Disposition: attachment; filename=$file_name");
header("Pragma: no-cache");
header("Expires: 0");