You could add a logical field to your products table that indicates if the thumbnail exists and same for a large images too. Storing a zero for NO & a one for YES in your indicator field. You could probably make that default to NO, and update to YES when a successful upload is completed for each record.
php:
<?php
if ($row_getProduct['thumbnail_exist']==0) {
?>
<img src="images/default_thumb.jpg" width="95" border="0">
<?php
} else {
?>
<img src="images/<?php echo $row_getProduct['thumbnail']; ?>" width="95" border="0">
<?php
}
?>
