I tried this but my commandlink disappeared. When i tried to find with firebug, it was there but with size 0 x 0 px.
.myNewButton {
width: 50px !important;
height: 50px !important;
background-image: url('/resources/img/e_menu_icons/x.png') !important ;
}
.myNewButton:hover {
width: 50px !important;
height: 50px !important;
background-image: url('/resources/img/e_menu_icons/e.png') !important ;
}
<p:commandLink styleClass="myNewButton" value=""
oncomplete="myDialog.show(); return false;"
action="#{myBean.actionMyAction()}">
</p:commandLink>