//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="/images/sharethis.png" /></a></span>');
//Tie customized button to ShareThis button functionality.
var element = document.getElementById("share");
object.attachButton(element);