if (window.addEventListener) window.addEventListener('load', setSidebarBackground, false);
if (window.attachEvent) window.attachEvent('onload', setSidebarBackground);

function setSidebarBackground() {
	var imageCount = 5;
	var imageNum = Math.floor(Math.random() * imageCount) + 1;
	if (document.getElementById('left_column')) document.getElementById('left_column').className = 'bg'+imageNum;
}