How to handle larger server side delays in a web application?

With this post I would like some advice on designing a certain web application. I'm not looking for code examples, just different approaches to a particular problem.

To make it simple, I'm directly going to the core of my question. :-)

I would like to create a webform with some text input fields. I would to submit the data to a webservice, which will handle the request ...

When processing the information from the user on the application server, it will generate a JPG preview image, to be displayed back in the users browsers. This process can run from 1 to 500 seconds (but hopefully not that long)...

How do I handle the client side of the application?

Do I submit the information using ajax, display a wheel, and poll every second to the webservice to check if the preview is ready or still pending ? Or is there a better approach on handling larger server side delays ?

Or is there some way to push back the preview to a certain user when it's ready ? (Just a wild thought)

I'm using Java and Glassfish ...

How do the pro's handle this type of situation ?

Any feedback would be greatly appreciated ...