Swapping out the ImageURL in an ASP Image Button using Java

So, I am building a "guided tour" of sorts for a product we are launching.  On my website, I have 6 imagebuttons that when clicked call this javascript to show/hide the divs I have the content that each one relates to.  



function toggleLayer( whichLayer )
{
  var elem, vis;
  var all = document.getElementsByTagName("div");
        // loop all the divs for this page and set them hidden
      for (var i=0; i<all.length; i++)
      {
            if( all[i].id.indexOf('pan_') != -1 )
            {
                  all[i].style.display = "none";
            }
      }
      
  if( document.getElementById ) // this is the way the standards work
    elem = document.getElementById( whichLayer );
  else if( document.all ) // this is the way old msie versions work
      elem = document.all[whichLayer];
  else if( document.layers ) // this is the way nn4 works
    elem = document.layers[whichLayer];
  vis = elem.style;
  // if the style.display value is blank we try to figure it out here
  if(vis.display==''&&elem.offsetWidth!=undefined&&elem.offsetHeight!=undefined)
    vis.display = (elem.offsetWidth!=0&&elem.offsetHeight!=0)?'block':'none';
  vis.display = (vis.display==''||vis.display=='block')?'none':'block';
 
}

However, to make sure they dont reset to their hidden state, I have this on all the buttons as well:

        search_CityZip.Attributes.Add("onclick", "return false;");  

To avoid a postback which resets the panels to their closed states.  What I'd like to do also, is to swap out the picture I have on each button which is currently a blue arrow, with a green arrow.  Sounds simple enough, but I can't do this server side, because the buttons don't cause a postback currently.  Any help or suggestions would be greatly appreciated!!
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