Modify:
In:
Home » Themes » YOURTHEME » g33k_thankyoulike.css
a.add_tyl_button span{
background-image: url(images/thankyoulike/tyl_add.png);
background-repeat: no-repeat;
font-weight: bold;
}
In:
Home » Themes » YOURTHEME » g33k_thankyoulike.css