images don't have anything to do with html/css

It depends - are you prototyping by making wireframes in an image editor or prototyping by coding up a rough working copy? If you're a designer and prefer mocking stuff up in Fireworks or Photoshop, then you should have the chops to build upon the gray boxes and text and layer in effects to add visual polish. If you're a coder and would rather create crude functional demos first, then you should have the html/css chops to layer in visual polish via background images or etc.

Either way, designer or coder, it helps to concentrate on the rough functionality first and then layer things in later, like typography, logos, icons, color schemes and other visual polish.

I'm giving a talk at Adobe in about a week and a half on the subject of rapid prototyping in Fireworks, I think there'll be a screencast or video going on, for anyone interested:

http://www.meetup.com/Fire-On-The-Bay/calendar/9583191/

And don't just take my word for it, Mark Boulton's got a great PDF book out with the same approach, if you're interested in improving your process as a designer:

http://www.fivesimplesteps.co.uk/