Question: I have a website where users each have their own profile page. Here they can upload a single image that acts as their avatar. This is the only image users can upload to the server across the whole site. There is no archive so it can be overwritten if a user wishes to update their avatar.

I have never had to do anything like this before so I would like to open it up and ask for a suitable, scalable option for this website.

My initial thought is to give each user's image a random name, a string 6-12 characters long. This way you couldn't build a script that just pulls every user's profile pic from a directory (eg 001.png, 002.png etc). My second thought is that there should be only be a certain amount of images per directory to make sure they can be retrieved quickly by the server.

There may well be other things I'm missing here, I'm not sure on exact details hence why I'm asking.

Answer: 
I would recommend storing the images on something like Amazon S3. Depending on how many pictures you're storing, serving images can really take a tow on your web server. S3 is scalable and with multi-zone deployments through CloudFront (Amazon's version of a CDN), you can really speed up this part of your service.
It's good idea to not overload single directory. Very often you can see that images are stored in hierarchy of folders according to theirs first few letters. An example of this is

```
b5dcv5.jpg -> /b/5/b5dcv5.jpg
bsgb0g.jpg -> /b/s/bsgb0g.jpg
a5dcbt.jpg -> /a/5/a5dcbt.jpg

```

and so on. I thing you got the principle. Advantage of this is to have access to and image in O(log N) when filenames are uniformly distributed instead of O(N) as it would be in single folder solution.