when to delete a temporary file ?

Hi, I've created a site where users can upload photos and specify which other users are allowed to view them using PHP session management. now I have alittle problem in the post-upload page, script creates a temporary copy of the uploaded image with a name the same as user's session ID, and puts a <img src="temp file..."> in the page for the user to view the uploaded photo and confirm it. now I don't know when to delete that temporary file ! because I don't know how long does the user's browser take to download the image. currently I delete it in the script coming up just after user clicks the confirm button, but what if the user close his/her browser before clicking the confirm button ? the file remains there for ever !! what's your suggestion ? Thanks Arash Dejkam