I am using the LESS styling language.
Consider the following CSS:
.side-bg
{
background:url(../img/layout/side-bg.jpg) top no-repeat;
}
Relative urls can be handled by the command line compiler, supposedly. There's probably some similar option you can set in the file watcher.
https://github.com/cloudhead/less.js/wiki/Command-Line-Usage
EDIT: There totally is. Just look: http://lesscss.org/usage/#command-line-usage-options
relativeUrls: true