Looking at how imgur serves its images. How come when I check network tab in Chrome, its sending this PNG and its only 387B?

Question

I'm looking at

http://i.imgur.com/mkaX2Eo.png

and checked the network tab in chrome, and this is what I see:

When I look at the response tab for that .png, this is what I see:

What is that? How do they send over something like that which is 387B and then render the image in browser?

What can I search for on google so I can set up something like that on my nodeJS api server?

Thanks