Which javascript code enables images to be updated ...

Check out the site dynamic drive. they have lots of scripts that you can use (and adapt) for your blog. Here is an example of a script that rotates images, html etc.

In javascript you can access an IMG HTML tag and change it's SRC property causing the browser to load a different image.

There are a couple of tutorials here which might help you. 

There is a sample code that does something like you want with JavaScript and random images which you can check out here. I haven't used it though. 

If you require further assistance search for the keywords "JavaScript images timer" in Google and you should find more results. 

I'm sure this is not what you had in mind, but thought you might find this interesting?

http://www.splitbrain.org/projects/monsterid

Rob

www.robinclapworthy.com