Different images getting saved to the same file??? (ImageJPEG)

I am creating a barcode using function ImageJPEG as below: ImageJPEG($picture,"./temp/graph.jpeg"); The image created is different according to the values informed by the user in a form. A file named 'graph.jpeg' is created in directory 'temp' and the image is shown to the user when he submits the form. Now, how can I be sure that some user is not going to have the image resulting from some other user's form submission shown at his browser if I always specify the same filename for the image created? ==> Do I have to create different image filenames, lets say by naming the image files after some combination of the user's Session ID or IP address? And then (if this is the case) how should I get rid of the files created on the hard disk? Thanks, Paulo