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.
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.