Image not picked from page template theme

My Kentico installation is running at: localhost:8012 I created a new page template for my page. In the Theme tab, I created following three folders:

css, images, fonts

I uploaded an image at: images/back.jpg

Three is a file named: css/custom.css which contains a line:

background: url('../images/back.jpg') no-repeat center center;

When I check the page, it says back.jpg not found and tries to find it at:

localhost:8012/images/back.jpg

This means, it is not using relative path. What can I do to fix the path?

Thanks