//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.'
},
            { offsetLeft: 194, offsetTop: -42 },
            { embeds: true },
            { button: false });
//Output your customized button
document.write('<span id="share" style="float:right; margin-right:10px;"><a href="javascript:void(0);"><img src="images/share_this_icon.gif" alt="Share This with a Friend" width="106" height="47" border="0"></a></span>');
//Tie customized button to ShareThis button functionality.
var element = document.getElementById("share");
object.attachButton(element);