The only help you could provide, and it will be minimally helpful, is to mimic the Rails directories,
public/stylesheets <--- put css here
public/images <-- put image files here, if you refer to them in css, use ../images/image.gif or whatever
But honestly, anything you do to make the rails programing easier will just add keystrokes to your work. The only thing that will really help is to keep references to your images in the '../images/' form.
Dreamweaver is fine for what you're doing. I'm the opposite of you, I'm the rails programmer who gets the raw HTML/CSS design. Honestly, if you do NOTHING to accommodate the rails programmer, you may add a few minutes of extra work for him, but it's not a big deal.
Joe got a job, on the day shift, at the Utility Muffin Research Kitchen, arrogantly twisting the sterile canvas snout of a fully charged icing anointment utensil.