Nonono, you've missed how it is supposed to work. This script invokes an external script to produce an image and save it to a temporary file. This script sends out an HTML which contains a link to this temporary file as an URL for an IMG tag. The browser is supposed to pick up the HTML and load the image in it.

To the OP: perhaps the time difference between the saving of the file and sending the URL to the browser is too short. Perhaps the server hasn't finished saving when the webserver tries to access the file. I don't know, it's just a wild guess, really.