Those are all different things that do need their own separate sites in my opinion. Here is my opinion on the matter.
Graphic Design = don't always mess with code, or web design.
Webmasters = deal more with setting up, domain management, and maintaining a website, and not web design. Most webmasters will install themes, not create them.
Just like there is a difference between web designer and web developer. The difference is one is good at web markup, and the other at web programming. I've seen web developers call themselves designers(when they have no artistic talent), and web designers call themselves web developers(even though they do not know programming languages like javascript/php/etc, only how to implement existing pre-written scripts(which is what a web master can do)).
It seems confusing, but it's not. They are all clearly separate things.
Stackoverflow only covers markup & development, but not design. See the difference now?
I do business and offer services as web designer, graphic designer, hosting, or setting-up/maintaining your site(webmaster role). Even though I can do all of that, I wouldn't call myself a developer. Nor would I say these roles/jobs are similar enough to keep it to one site, because it increases the odds of having my questions closed.