@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

#stylish-panel {
  background-image: url("your image here") !important;
  background-position: center center !important;
  background-repeat: no-repeat !important;
  background-color: transparent !important;
/*optional spacing stuff */
  margin-right: 4px !important;
  margin-left: 6px !important;
}
#stylish-panel > .statusbarpanel-icon {
  visibility: hidden !important;
  background-color: transparent !important;
}