AddEventHandler("iblock", "OnBeforeIBlockElementUpdate", "DoNotUpdate");
function DoNotUpdate(&$arFields)
{
    if ($_REQUEST['mode'] == 'import') {
        if (isset($_GET['type'], $_GET['mode']) && $_GET['type'] === 'catalog' && $_GET['mode'] === 'import') {
            unset($arFields['PREVIEW_PICTURE']);
            unset($arFields['DETAIL_PICTURE']);
        }
    }
}