AddEventHandler("iblock", "OnBeforeIBlockElementUpdate", "setCode");
function setCode(&$arFields)
{
     if ($arFields["PREVIEW_PICTURE"]["name"]!='' and $arFields["PREVIEW_PICTURE"]["old_file"]!='' )
    {
        //пошло обновление
        $old_md5 = md5_file($_SERVER['DOCUMENT_ROOT'].CFile::GetPath($arFields["PREVIEW_PICTURE"]["old_file"]));
        $new_md5 = md5_file($arFields["PREVIEW_PICTURE"]["tmp_name"]);

        if($old_md5==$new_md5)
        {
            //картинка не изменилась
            $arFields["PREVIEW_PICTURE"]=array(
                'name' =>'',
                'type' =>'',
                'tmp_name' =>'',
                'error' => 0,
                'size' => 0,
                'description' => '',
                'MODULE_ID'=> 'iblock',
                'old_file' => $arFields["PREVIEW_PICTURE"]["old_file"]
            );
        }
    }

    if ($arFields["DETAIL_PICTURE"]["name"]!='' and $arFields["DETAIL_PICTURE"]["old_file"]!='' )
    {
        //пошло обновление
        $old_md5 = md5_file($_SERVER['DOCUMENT_ROOT'].CFile::GetPath($arFields["DETAIL_PICTURE"]["old_file"]));
        $new_md5 = md5_file($arFields["DETAIL_PICTURE"]["tmp_name"]);

        if($old_md5==$new_md5)
        {
            //картинка не изменилась
            $arFields["DETAIL_PICTURE"]=array(
                'name' =>'',
                'type' =>'',
                'tmp_name' =>'',
                'error' => 0,
                'size' => 0,
                'description' => '',
                'MODULE_ID'=> 'iblock',
                'old_file' => $arFields["DETAIL_PICTURE"]["old_file"]
            );
        }
    }
}