SnowFlakeTango, you wouldn't be able to get this effect easily in iWeb because iWeb has a fixed width for the page that is defined in the code. The closest thing you could get to doing this is make a simple html page in something like coda or text edit. And put in some basic code like this to create the page and add images:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
</head>
<body>
<img src="http://www.yoursite.com/Link/ToYour/ImageHere.jpg" alt="“name" />
<img src="http://www.yoursite.com/Link/ToYour/ImageHere.jpg" alt="“name" />
<img src="http://www.yoursite.com/Link/ToYour/ImageHere.jpg" alt="“name" />
<img src="http://www.yoursite.com/Link/ToYour/ImageHere.jpg" alt="“name" />
<img src="http://www.yoursite.com/Link/ToYour/ImageHere.jpg" alt="“name" />
</body>
</html>
Then you'll see that page will have the desired effect, then if you needed that into iWeb, just put that page online and iFrame it into iWeb and make sure the scrollbars code option is set to "yes".
The other option you could use is Airtight Interactive has a great slideshow that has a similar effect to that which is totally free. I'd say go get it, modify it with your images and iFrame it into iWeb. Here's the link: http://www.simpleviewer.net/autoviewer/
Hope that helps.