This is CS50 OpenCourseWare. Courses - Google Code University - Google Code. 3+ Ways to Preload Images Using CSS, JavaScript, or Ajax. Preloading images is a great way to improve the user experience.
When images are preloaded in the browser, the visitor can surf around your site and enjoy extremely faster loading times. This is especially beneficial for photo galleries and other image-heavy sites where you want to deliver the goods as quickly and seamlessly as possible. Google HTML/CSS/Javascript from the Ground Up Class: Introduction.