Git does work for the design files (however less well than with the text the programmers work with).
I'd say generally speaking, it works better if you do design and cutups for a programmer who then implements a fork. He then submits the fork to the project to be merged in to the main project if the rest of the implementors agree.
Gotta find a front end programmer who likes you and thinks you're doing the right thing for that to pan out well.