function changeImageOn_handler ( e ) {
	var img = getEventSource( e );
	if ( img ) {
		var imgOn = img.src.replace( '_off', '_on' );
		changeImage( img, imgOn );
	}
}
function changeImageOff_handler ( e ) {
	var img = getEventSource( e );
	if ( img ) {
		var imgOff = img.src.replace( '_on', '_off' );
		changeImage( img, imgOff );
	}
}
function changeImage ( imgElement, srcUrl ) {
	imgElement.src = srcUrl;
}