I think you should do the promos yourself as it is part of the product making process. It's best that you avoid other people doing the work because everyone puts a different amount of effort into their promo images. They may want more than just a credit and a copy of the product. On the other hand, some people may do it for free just to help you out.

However, if you are unhappy with the work the other person does, how much time do you think they will want to invest in making the changes? (especially if you want alot changed).

If you suck at making promos, I suggest you go through the marketplace and see what promos inspire you. Learn from how other people make their promos and add your own style in your own promos.

It's important that you know how to do this as good promos can make or break your product.