I agree with Anita, that it's probably a good idea to have it in your contact that you use subcontractors for that particular task. I don't think you necessarily have to mention on your website and other advertising who will be doing what service. I don't feel it would be hiding anything not saying that. But I also don't see a problem putting on your site that you subcontract out the design work to a graphic designer (or something like that) if it would make you feel better.
Now when it comes down to talking with a PC about the design work, you would probably need to tell them then unless you would be the one talking with them about the project details, etc. My husband has been trying to get me to list accounting on my site for him to do, so I've been looking at the same questions. I would not feel knowledgeable enough to discuss and meet with clients on accounting stuff, as I know so little in that area, so he would have to be the one working directly with the client in this area.
I'm a sole proprietor, too.
And congratulations!!