No quite sure what you mean by empty space.. but
depending on how you generate your thumbnail..
here is some quasi-code that may help..using php
<?
$ImageInfo = getimagesize("imagedir/yourthumbnail");
$image_width=$ImageInfo[0];
$image_height=$ImageInfo[1];
?>
<table cellpadding=2>
<tr>
<td width="<?= $image_width?>" height="<?= $image_height ?>" valign="center" align="center">
[img]yourthumbnail[/img]" height="<?= $image_height ?>" alt="some optimized text">
</td>
</tr>
</table>