Hello,
You could simply just add the alt"" with empty entries. If you were just looking to make it xhtml compliant anyways. You could even just give all image attachments the name of your site. Example:
Code:
<img class="attach" src="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]&stc=1&d=$attachment[dateline]" border="0" alt="YOUR_SITE_NAME_HERE - views: $attachment[counter] - size: $attachment[filesize] - id: $attachment[attachmentid]" />