HTTP Response with Multiple Layout Images in one PNG Image

I was playing around inspecting amazon.com http responses and I noticed this HERE as you can see the images used in the page layout are contained in one PNG image,

• Is this a known approach ?

• As I imagine, The JavaScript on the page will place each part of the image in the right place accoarding to a specific X&Y positions in that image, Is this really what's happening ?

• Why are they doing this instead of requesting each image ? Is a new request more expensive than in-browser image processing ? (if my above assumption is correct)

• Suggestions for further reading on this thing is much appreciated

Thanks!