I want a
Try this handy tool, it'll generate alpha colours for you, you're basically trying to use an rgba value, but of course gotta be aware that this will only be supported by newer browsers...
http://www.colorzilla.com/gradient-editor/
This is what it came up with for a black to 100% alpha gradient: background: url(IMAGE_URL) linear-gradient(to bottom, rgba(0,0,0,1) 0%,rgba(255,255,255,0) 100%);