Image url in Css file under stylesheets under public Rails 2.3

Image url in Css file under stylesheets under public Rails 2.3

I am working with Rails 2.3.5 and I have images in public/images/ which
are added into css file named as custom.css
#cssmenu ul {
background: url(nav-bg.png) repeat-x 0px 4px;
height: 69px;
}
How can i make this read the image which in inside public/images ? I have
tried this but it did not work
#cssmenu ul {
background: url(<%= asset_path '/images/nav-bg.png' %>) repeat-x 0px 4px;
height: 69px;
}