Във head-a поставяш следното :
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script type="text/javascript" src="scrolltopcontrol.js"></script>
а в scrolltopcontrol.js си променяш
controlHTML: '<img src="../images/up.png" style="width:48px; height:48px" />', //HTML for control, which is auto wrapped in DIV w/ ID="topcontrol"
пътя до картинката ..