editing lib-common.php if you are brave--i'm not--is probably the way to go here. you would be interested in "function COM_article." you could probably get away with modifying this function and putting it into your lib-custom.php or maybe your theme's functions.inc... i'm not sure.
look around line 475 for a starting place.
Text Formatted Code
$topicimage = '<img align="' . $_CONF['article_image_align']
. '" src="' . $imagebase . $T['imageurl'] . '" alt="'
. $topicname . '" title="' . $topicname . '" border="0">';
$article->set_var( 'story_anchortag_and_image', '<a href="'
. $topicurl . '">' . $topicimage . '</a>' );