//Create your sharelet with desired properties and set button element to false
var object = SHARETHIS.addEntry({ title:'images.tranquilli.org', summary: 'Someone wanted to share this image gallery with you.\\n\\nUn utente ha condiviso questa galleria con te.'},{button:false},{popup:true});
//Output your customized button
document.write('<span id="share"><a href="javascript:void(0);"><IMG SRC=/ico/share-icon-24x24.png  ALIGN=ABSMIDDLE BORDER=0></a></span>');
//Tie customized button to ShareThis button functionality.
var element = document.getElementById("share");
object.attachButton(element);