I have an image I would like displayed on an Apple watch but I'm brand new to mobile app marketing.
I'm hoping I could find a place which might point me in the right direction.
Any freelance website, you can try (I feel like a Yoda suddenly!)
Most of them cost nothing for posting a job and you can see what kind of freelancers and costs you'll get.
I don't think there's a market yet specifically for Apple Watch on freelance and a mockup can be done by designers who don't code (although, it's better to find one who knows what he/she's doing, yes), so try any.
Honestly, you might need to plan some budget for the trial and errors unless you can give specific instructions and know what you want; there's lot of cheap designers/freelancers who will probably agree to build you a toaster for $5 if you'd ask them, but you might not be able to do anything with it besides burning the house down. So it's good if you do some researches, ideally ask the people who will build that app for you how they want their files and what they don't want or can't be done. And have a plan, the same way you'd need one for a website or any other app.
I suggest you find the dev/coder first, put them on hold after they told you their requirements, and then find the designer. You can always show a draft to the coders, and get a real mockup later.
I don't know the good freelance sites much but have a look at Elance (Upwork) and check the Freelancing stack, you might see some other good sites and complete your research there or find some clues where to look https://freelancing.stackexchange.com/