The site you mentioned is not using a random set of images. They are coded into the html side of the aspx page.
You could place an asp Image control on your page. Then on the page's Page_Load function set the image to a random picture of your set.
protected void Page_Load(object sender, EventArgs e)
{
this.Image1.ImageUrl = "~/images/random3.jpg";
}
You have different options on where to store the image set data. You could use a database and store the urls in a table. This would allow to use the built-in Random function found in SQL. Or you can save a XML file to the server, load that then use the Random .Net class to pick one of your xml nodes.
Personally i would recommend the Database solution.
EDIT: Because the server session is destroyed after 20mins you may want to look at using cookies so you can see the last random image they saw.