I think the code would be more like:
<div class="big-images">
<div><span class='zoom' id='ex1'><img src="libra/images/<?php echo($rsProduct->getColumnVal("ProductSKU")); ?>.jpg" /></span>
</div>
</div>
<div class="thumbs">
<?php
$letters = range('A', 'Z');
foreach ($letters as $letter) {
if (file_exists("libra/images/".$rsProduct->getColumnVal("ProductSKU") . "-".$letter.".jpg")) {
?>
<div><img src="libra/images/<?php echo($rsProduct->getColumnVal("ProductSKU") . "-".$letter) ?>.jpg" /></div>
<?php
}
}
?>
</div>