function swap_image (id, type, extension) {
	if (typeof extension != 'undefined') {
		extension = '.' + extension;
	} else {
		extension = '.gif';
	}

	path = '/_images/boton_' + id;

	path += (type == 'on') ? '_on':'';
	path += extension;
	
	var item = document.getElementById (id);
	item.style.backgroundImage = 'url(' + path + ')';
}