I would suspect that the command is not timing out, but the browser connection is. If you send nothing to the browser for a while, it assumes that you're not going to send anything.

One thing to try would be to have the script kick of image generation to a temporary location. Then go to a new page that reloads every x seconds (where x should be 15-60 (ish)). Check for the image being generated on the new page, and display it (in a new page perhaps) when done.

Don't forget to clean up older images or you'll fill up your disk. You could do this with a script called by a cron job. Leave enough time for the image to be downloaded.

Colin Scott
If you build it, they will be dumb...