Выводим фотку по его ID

$str_IMAGE_ID - ID фото
echo CFile::ShowImage($str_IMAGE_ID, 200, 200, "border=0", "", true);

или

$image1 = intval($ar_res["PREVIEW_PICTURE"])<=0 ? $ar_res["DETAIL_PICTURE"] : $ar_res["PREVIEW_PICTURE"];
$image2 = intval($ar_res["DETAIL_PICTURE"])<=0 ? $ar_res["PREVIEW_PICTURE"] : $ar_res["DETAIL_PICTURE"];
echo CFile::Show2Images($image1, $image2, 400, 400, "hspace='0' vspace='0' border='0' title='".$arElement["NAME"]."'", true);