I think its due to the following and your atomicon gallery they both use jquery so you'll have to disable the slider if you want to use the gallery most likely.
<script type="text/javascript" src="http://tsionakat.gr/modules/mod_dMark_jSlider/jSlider/jquery.min.js"></script>
<script type="text/javascript" src="http://tsionakat.gr/modules/mod_dMark_jSlider/jSlider/jslider.js"></script>
<script type="text/javascript">
jQuery(document).ready(function() {
jQuery('#slider1').s3Slider({
timeOut: 2500 });
});
</script>