What I am talking about is like this website :
http://www.ernesthemingwaycollection.com
It has a static wallpaper and a set of images that change from page to page, I want to implement a similar way of displaying random images from a set of images using ASP.NET.
EDIT : I want the image to stay the same in a session, and change from a session to another.
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.
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.