Swap photos randomly

I did a search on this site but am not even sure what you call the procedure I need.

Every week I do a bulletin of news for our intranet and add a new picture and delete the one that was there. Ideally I would like to have a directory of pictures and then let it sequentially go through the list and change it each time someone loads a page.

How do I do this. (I might have about 100 pictures). Am using DWMX and it is a simple html intranet. Many people who are on the intranet have old computers so it would be nice to take their situation into account if at all possible!

Thanks