I'm looking for suggestions and ideas how you might program a tool that automatically captures website screenshots, such that you could incorporate them into a website directory, etc.

To give an idea of what I'm looking to do, try out this site I came across:
http://webshots.velocitysc.com/WebshotSandbox.aspx

Now my question is, how is such a thing programmed? Let's say I wanted to develop a website directory where users could submit their websites/URLs, then on some periodic basis, a script went out, took snapshots of the websites, and incorporated them automatically into a directory.

How would you go about translating the site's html into a visual graphic .jpg file?  Any ideas?

It's an interesting programming problem to solve. There are probably some scripts out there if I look hard enough. I am familiar with the PHP image manipulation functions. And, I would guess ASP and other languages have similar functions to create, edit and save image files. However, I am not sure how you translate someone's souce HTML code into an image file. It seems you'd need a whole HTML/CSS parsing engine to do that....i.e., the core of a web browser.

And you'd have to worry about HTML 4.01 loose, strict, XHTML 1.0, etc. You probably wouldn't want to have to re-code an entire web browser, HTML-rendering engine. Hmmm... Would the answer be to somehow use a browser from a command-line, shell-command, etc? Or, could you get your hands on a freeware version of a browser-core that you could run from inside a PHP/ASP script? Hmmm... just thinking out loud here....

You can download a command line utility that will allow you to take screenshots of websites at http://www.nathanm.com/index.php/webshot-website-screenshot-capturing/