Hi ragnarorn !
it is posible, you can do it following some steps below :
1. Install ja slideshow in to your site.
2. assign for ja slideshow to position top, unpublish ja_newsflash module.
3.Open index.php file in templates/ja_iris folder , find following code section at about line 103 :
<?php if ($this->countModules('top')) { ?>
<!-- BEGIN: TOP SPOTLIGHT -->
<div id="ja-topsl" style="background: url(<?php echo $tmpTools->templateurl();?>/images/headers/<?php echo $tmpTools->getRandomImage(dirname(__FILE__).'/images/headers/'); ?>) no-repeat right top;">
<jdoc:include type="modules" name="top" style="xhtml"/>
</div>
<!-- END: TOP SPOTLIGHT -->
<?php } ?>
change to :
<?php if ($this->countModules('top')) { ?>
<!-- BEGIN: TOP SPOTLIGHT -->
<div id="ja-topsl" >
<jdoc:include type="modules" name="top" style="xhtml"/>
</div>
<!-- END: TOP SPOTLIGHT -->
<?php } ?>