How to generate a unique hash for a URL?

Question

Given these two images from twitter.

http://a3.twimg.com/profile_images/130500759/lowres_profilepic.jpg
http://a1.twimg.com/profile_images/58079916/lowres_pr         


        

Answer 1

One of the key concepts of a URL is that it is unique. Why not use it?

Every algorithm that shortens the info, can produce collisions. Maybe unlikely, but possible nevertheless