Image swap and class change in layout

I’m still a newbie and can’t figure out where and also how to do this:
I just have a simple navigation bar on the right that I would like to
keep seperate from the other layouts that looks pretty much like:



My images simply add “_current” to the end of the image name to signify
that is the current page.

I’m curious how write this in Rails so I can keep it in a seperate
.rhtml file and still switch image files as each page is viewed (e.g.
about is selected and the image would change to about_current.gif and
home would revert back to home.gif, as well as changing the a href to
class=“current”), and how would I call this from the controller’s main
layout.

Thanks ahead for the help!