Python/Django: how to get files fastest (based on path and name)
The access time for an individual file are not affected by the quantity of files in the same directory. 
running ls -l on a directory with more files in it will take longer of course. Same as viewing that directory in the file browser. Of course it might be easier to work with these images if you store them in a subdirectory defined by the user's name. But that just depends on what you are going to doing with them. There is no technical reason to do so. 
Think about it like this. The full path to the image file (/srv/site/images/my_pony.jpg) is the actual address of the file. Your web server process looks there, and returns any data it finds or a 404 if there is nothing. What it doesn't do is list all the files in /srv/site/images and look through that list to see if it contains an item called my_pony.jpg.
My website users can upload image files, which then need to be found whenever they are to be displayed on a page (using src = ""). Currently, I put all images into one directory. What if there are many files - is it slow to find the right file? Are they indexed? Should I create subdirectories instead? 
I use Python/Django. Everything is on webfaction.