How to do load detailed image as a HTML background?

How do people like Bing and Google get such hi-def background images to load so quickly on their websites?

What tricks do they use to make such开发者_运维知识库 big background images feel so light-weight to load?

I have a crappy internet connection so I know when something loads relatively quickly.

There is no trick. Bing's background is usually 80-100kB with a good image compression you can get that. Additionally they do a fade-in effect when the whole image is loaded which makes it look better. Finally their CDN ensures that it loads quickly all over the world.

The images they use are normal JPG files, nothing special to it.

The reason why it feels so fast is probably the delivery. Big sites use Content Delivery Networks, which most often have servers very close to you physically. Those servers have very fast response times and are specialized on serving static content, making full use of the available bandwidth.