//Create your sharelet with desired properties and set button element to false
var object = SHARETHIS.addEntry({
title:'share',
summary: 'Sharing is good for the soul.'},
{button:false});
//Output your customized button
document.write('<span id="share"><a href="javascript:void(0);"><img src="http://www.drmoliver.com/basic_graphics/share-icon-15x15.png" border="0"/></a></span>');
//Tie customized button to ShareThis button functionality.
var element = document.getElementById("share");
object.attachButton(element);

function MM_openBrWindow(theURL,winName,features) { //v2.0
  window.open(theURL,winName,features);
  }