I think that the simplest way to do this is to edit the template for the tab-pane, to put a clearing line-break after the image, then the text will be forced to start on the line below.
To do this, edit the module file tmpl/tabpane.php, change lines 16 to 17 from:-
echo $item->imageTag;
echo $item->text;
to
echo $item->imageTag;
echo '<br style="clear:both;" />';
echo $item->text;
You can also do this through template overriding rather than editing the module template file, see
www.spiralscripts.co.uk/Joomla-Tips/usin...ing-with-joomla.html 
