$(document).ready(function() {
   $( "#navigation_type li a img" ).mouseover( switchOn );
   $( "#navigation_type li a img" ).mouseout( switchOff );
} );

function switchOff() {
	this.src = this.src.replace( "_on", "_off" );
}

function switchOn() {
	this.src = this.src.replace( "_off", "_on" );
}