Hi, for largers sites the user directory could be replaced by subdirectories in the way:
filename: dupa.jpg
md5sum: 9b38f4daa67682b389b586e6376d03f7
path to file: albums/userpics/9/b/9b38f4daa67682b389b586e6376d03f7.jpg
Maintaining md5sum of the file helps also in consistency check of it.