It is important to include the width and height attributes in each image tag.  The mark-up is valid without a height and width attribute, but it is best practice to include them.

You can include them by adding the width and height attributes to the images in the following file: modules\mod_nice_social_bookmark\mod_nice_social_bookmark.php

Back up first!

The code for the whole file is below:

<?php

// no direct access

//defined( '_VALID_MOS' ) or die( 'Restricted access' );

$url = "http://".$_SERVER['HTTP_HOST'] . getenv('REQUEST_URI'); 

$rss_url = "http://".$_SERVER['HTTP_HOST'];

$isize = $params->get('isize');

$iset = $params->get('iset');

$iposition = $params->get('iposition');

$document = JFactory::getDocument();

$opac = $params->get('opac');

if ($opac=="yes"){

 $document->addStyleSheet('modules/mod_nice_social_bookmark/css/nsb-opac.css');

 }

 elseif ($opac=="invert"){$document->addStyleSheet('modules/mod_nice_social_bookmark/css/nsb-opac-inv.css');

 }

 else{$document->addStyleSheet('modules/mod_nice_social_bookmark/css/nsb.css');

 }

$twlink = $params->get('twlink');

$fblink = $params->get('fblink');

$mslink = $params->get('mslink');

$rsslink = $params->get('rsslink');

echo '<div align="'.$iposition.'">';
$tt = $params->get('s1', 'yes');
if ($tt == "yes"){ if ($fblink == "")
echo '<a id="l1" target="_blank" rel="nofollow" href="http://www.facebook.com/sharer.php?u='.$url.'&title="><img title="Facebook" border="0" src="modules/mod_nice_social_bookmark/icons/facebook_'.$iset.'_'.$isize.'.png" alt="Facebook" width="32px" height="32px"  /></a>&nbsp;';
else echo '<a id="l1" target="_blank" rel="nofollow" href="http://'.$fblink.'"><img title="Facebook" border="0" src="modules/mod_nice_social_bookmark/icons/facebook_'.$iset.'_'.$isize.'.png" alt="Facebook" width="32px" height="32px"/></a>&nbsp;';}
$tt = $params->get('s2', 'yes');
if ($tt == "yes"){ if ($mslink == "")
echo '<a id="l2" target="_blank" rel="nofollow" href="http://www.myspace.com/Modules/PostTo/Pages/?l=3&amp;u='.$url.'&amp;title="><img title="MySpace" border="0" src="modules/mod_nice_social_bookmark/icons/myspace_'.$iset.'_'.$isize.'.png" alt="MySpace"  width="32px" height="32px" /></a>&nbsp;';
else echo '<a id="l2" target="_blank" rel="nofollow" href="http://'.$mslink.'"><img title="MySpace" border="0" src="modules/mod_nice_social_bookmark/icons/myspace_'.$iset.'_'.$isize.'.png" alt="MySpace" width="32px" height="32px" /></a>&nbsp;';}
$tt = $params->get('s3', 'yes');
if ($tt == "yes"){ if ($twlink == "")
echo '<a id="l3" target="_blank" rel="nofollow" href="http://twitter.com/home?status='.$url.'&amp;title="><img title="Twitter" border="0" src="modules/mod_nice_social_bookmark/icons/twitter_'.$iset.'_'.$isize.'.png" alt="Twitter" width="32px" height="32px" /></a>&nbsp;';
else echo '<a id="l3" target="_blank" rel="nofollow" href="http://'.$twlink.'"><img title="Twitter" border="0" src="modules/mod_nice_social_bookmark/icons/twitter_'.$iset.'_'.$isize.'.png" alt="Twitter" width="32px" height="32px"  /></a>&nbsp;';}
$tt = $params->get('s4', 'yes');
if ($tt == "yes")echo '<a id="l4" target="_blank" rel="nofollow" href="http://digg.com/submit?phase=2&amp;url='.$url.'&amp;title="><img title="Digg" border="0" src="modules/mod_nice_social_bookmark/icons/digg_'.$iset.'_'.$isize.'.png" alt="Digg" width="32px" height="32px" /></a>&nbsp;';
$tt = $params->get('s5', 'yes');
if ($tt == "yes")echo '<a id="l5" target="_blank" rel="nofollow" href="http://del.icio.us/post?url='.$url.'&amp;title="><img title="Delicious" border="0" src="modules/mod_nice_social_bookmark/icons/delicious_'.$iset.'_'.$isize.'.png" alt="Delicious" width="32px" height="32px" /></a>&nbsp;';
$tt = $params->get('s6', 'yes');
if ($tt == "yes")echo '<a id="l6" target="_blank" rel="nofollow" href="http://www.stumbleupon.com/submit?url='.$url.'&amp;title="><img title="Stumbleupon" border="0" src="modules/mod_nice_social_bookmark/icons/stumbleupon_'.$iset.'_'.$isize.'.png" alt="Stumbleupon" width="32px" height="32px" /></a>&nbsp;';
$tt = $params->get('s7', 'yes');
if ($tt == "yes")echo '<a id="l7" target="_blank" rel="nofollow" href="http://www.google.com/bookmarks/mark?op=edit&amp;bkmk='.$url.'&amp;title="><img title="Google Bookmarks" border="0" src="modules/mod_nice_social_bookmark/icons/google_'.$iset.'_'.$isize.'.png" alt="Google Bookmarks" width="32px" height="32px" /></a>&nbsp;';
$tt = $params->get('s8', 'yes');
if ($tt == "yes"){if ($rsslink == "")
echo '<a id="l8" target="_blank" rel="nofollow" href="'.$rss_url.'/index.php?format=feed&amp;type=rss&amp;title="><img title="RSS Feed" border="0" src="modules/mod_nice_social_bookmark/icons/rss_'.$iset.'_'.$isize.'.png" alt="RSS Feed" width="32px" height="32px" /></a>&nbsp;';
else echo '<a id="l8" target="_blank" rel="nofollow" href="'.$rsslink.'"><img title="RSS Feed" border="0" src="modules/mod_nice_social_bookmark/icons/rss_'.$iset.'_'.$isize.'.png" alt="RSS Feed" width="32px" height="32px" /></a>&nbsp;';}
echo '</div><div style="clear:both;"></div>';
?>

Hope this help.