[?] Deleting user created resources after user leaves site

[eluser]carvingCode[/eluser]
In the app I'm currently working on, I use 'jpgraph' to create graphs. Works great - highly recommended.

In this particular application, users set up searches by varying criteria and graphs are created using that criteria. So there will be many different graph files created.

Problem: What's the best way to handle deleting images after a user is no longer on the site? Users do not create accounts on this site. Is it a matter of setting up a cron job to run at a particular time? Or is there a way to do this using sessions?

TIA

Randy