function image_session() {
	var user_agent = navigator.userAgent;
	var firefox = false;
	if (user_agent.indexOf('firefox')) {
		firefox = true;
	}
	if (document.documentElement.offsetHeight != null && document.documentElement.clientHeight != null) {
		var available_height_1 = document.documentElement.offsetHeight;
		var available_height_2 = document.documentElement.clientHeight;
		if (firefox) {
			var available_height = available_height_2;
		} else {
			var available_height = available_height_1;
		}
		AjaxAPI.getText (
			site_root + 'ajax/image_session.php?available_height='+available_height,
			function (responseText)
			{	
			}
		);
	}
}