You can not set absolute path
from your locally. you can set absolute path only live web page. following way,
BODY { background-image: url(http://www.bg.com/pinkish.gif) }
another way set relative path
. I assume your root Website folder
__My Website
|__Files
|__images.jpg
|__welcome.css
|__welcome.html
It's relative to the style sheet, but I'd recommend making the urls relative to your url, base on above strutted set relative path on following way,
background-image:url('Files/images.jpg');