How does Instagram name files

Question

How does Instagram name files, are the file names random? based on some sort of hash? or is there some meaning to them? for example

http://distilleryimage2.instagram.com/21a9ca729bf511e2985c22000a1f9ad3_7.jpg

Answer 1

Good answer - The blog post is out of date now as their naming has changed, but it's probably still based on MD5.

Answer 2

After further inspection, I think it's just an MD5 hash or something followed by _# for the image size