Convert a web page to an image (thumbnail)

Possible duplicate:
How to export entire page or html content using Highcharts and not just a chart?

I would like to be able to generate a thumbnail from a given web address on the fly (and store it for future use). This needs to be done on the server (linux) without using too many resources.

Alternatively, is there a way to make it "invisible" on the client and then upload the screenshot to the server via AJAX cal or whatever?

Thank.