CopyDetailToPreview

CModule::IncludeModule('iblock');
$res = CIBlockElement::GetList(Array(), array('IBLOCK_ID' => 80));
while($ob = $res->GetNext())
{
    $el = new CIBlockElement;
    $arLoadProductArray = array("PREVIEW_PICTURE" => CFile::MakeFileArray(CFile::GetPath($ob["DETAIL_PICTURE"])));
    $test = $el->Update($ob['ID'], $arLoadProductArray);
    print_r($test);
}