Add a second hidden form element next to the WADADeleteRecordID one:
<input type="hidden" name="WADADeleteRecordID" id="WADADeleteRecordID" value="<?php echo($row_WADAtbSlides["id"]); ?>" />
<input type="hidden" name="WADADeleteFileName" id="WADADeleteFileName" value="<?php echo($row_WADAtbSlides["photo"]); ?>" />
edit the delete button onclick to pass the file name to the WADADeleteFileName form element
<input type="button" class="formButton ResultsPageButton unstyled DeleteButton" value="" onclick="document.getElementById('WADADeleteRecordID').value=<?php echo($row_WADAtbSlides['id']); ?>;document.getElementById('WADADeleteFileName').value=<?php echo($row_WADAtbSlides['photo']); ?>;document.getElementById('deleteBox').style.display = 'block';document.getElementById('deleteMessage').style.display = 'table';" />
then edit the delete file behavior, and bind the file name to the WADADeleteFileName form element.