Think about what part of the design you want to outsource precisely: just the way it looks or more?
This is relevant because it depends on whether you want someone to give you any of the following things:
- A set of one or more "pictures" of the different (types of) pages on the site.
- A layered representation of the above that incorporates resizing/variation.
- A static version of a basic site in HTML.
Based on that decision it will make more sense to work in parallel (the top option is easy to develop in parallel for, the bottom is more difficult (at least, requires careful planning)).