<h:commandButton title="Hola" image="/resources/images/boton/16x16/list.png"/>
.ui-icon-excel {
background: url("images/toexcel.gif") no-repeat top left !important;;
}
<p:commandButton type="submit"
id="cr1001_command_toexcel"
image="ui-icon-excel"
action="#{pc_Cr1001.doCr1001_command_toexcelAction}" ajax="false">
</p:commandButton>
<h:commandButton image="images/toexcel.gif"/>