1. Log in to your dashboard--> layout- -> Edit HTML

2. Click on "Expand Widget Templates"

3. Search "Ctrl+F" and find ]]></b:skin>
  
#backtotop {
padding:5px;
position:fixed;
bottom:10px;right:10px;
cursor:pointer;
}

4. Search for </body> and before it paste the next code: 

<a href="#" id="backtotop"><img src="http://img171.imageshack.us/img171/1494/topbm.png" alt="back to top" /></a>

change the "http://img171.imageshack.us/img171/1494/topbm.png" with the link to your own image.

5. Now find </head> and before it paste the following code: 

<script src='http://ajax.googleapis.com/ajax/libs/jquery/1.3/jquery.min.js'
&lt;type=&#39;text/javascript&#39;&gt;&lt;/script&gt;
&lt;script language=&quot;javascript&quot;&gt;
$(window).ready(function(){
$(&#39;#backtotop&#39;).click(function(e){
e.preventDefault();
$(&#39;html, body&#39;).animate({scrollTop:0}, &#39;slow&#39;);
});
});
&lt;/script&gt;


Save the template and see the result. You can see how it works on the right side of my site.