@phppup, it doesn't matter how the files are stored on disk unless your users are just loading up that directory and viewing the files directly.
Store them on disk however you want and just track which files belong to which user in your database then present them to the users that way.
You can't really sequence them how you want unless you know ahead of time how many photos each user will be uploading. Say you decide to just +10 for each user so Linda is 1-10, You're 11-20 and Ricky is 21-30. So everyone uploads one photo and now you have Concert_1.jpg, Concert_11.jpg, Concert_21.jpg like you want. But then Linda uploads 20 photos not 10? You have a problem, your files are no longer grouped nicely.
Forgot about trying to match the physical storage to some sort of order like that. Just store them. Handle the sorting/ordering in the DB.