Edit: Kodlarda Eksik var.. Yeniden Düzenliyorum..

ilk kodlara checkbox ekledim

<?
while($row = mysql_fetch_array($sql)){
?>
<div style="float:left; padding:11px; text-align:center;">
<img src="https://www.r10.net/images/pictures/biger/<? echo $row['picture'];?>" width="100" height="76" style="border:1px solid #ccc; "><br />

<input type="button" name="yeni" value="Resmi Sil" style="width:102px; background-color:#fff;" class="ButtonStyle" onClick="javascript:location='sil.php?id=<? echo $row['id']."&picture=".$row['picture']."&masterid=".$_POST['masterid'];?>';">
<input type="checkbox" name="resimsilx" />

</div>
<? } ?>

<?php
include_once "ayar.php";
$say = mysql_num_rows ($sql);
if($_GET['confirm']=="true"){
For ($i=0; $i <= $say;$i++) {
$sql = mysql_query("delete from `pictures` where `id`='".$_GET['id'][$i]."'")or die(mysql_error());
unlink("../images/resim/buyuk/".$_GET['picture'][$i]."");
}
?>

bu kodları denermisin..