Page load times are important. Amazon insiders estimate that every 100 ms increase in latency costs Amazon roughly %1 of profit.
Simply put: visitors hate slow sites, so don't make them wait.
Unfortunately, many web sites, including this one up until recently, are slowed down by inefficiently encoded images. Note that there are many other components to page load times and if you're looking to optimize your web site you should analyze and understand all of them. But today we're just going to focus on the images.