Graphics Designers are just like any other workers..The more details they have the better they can perform. I will break down your question into 2...
1) What to look for in graphics designers...
This is very tricky...everyone wants a good designer and wants to pay cheap prices. In my experience "good" and "cheap" do not go together..so you need to pick one..ask to see their portfolio and see what type of designs have they worked with. Find out how many revisions they can do. How fast they can deliver. Will they also code the design into html or not?
2) How to convey your ideas and give details about your projects..
I do this in 3 steps normally.. a) provide a through and detailed spec sheet about each and every page. Don't forget to include drawings and screenshots. Screenshots doesn't have to be of entire page. They can just be pieces of other sites you found. Remember, a picture is worth a thousand words and in this case where you are working with a designer it is very true...b) include a list of other websites and write some notes about what you like and what you dislike..this gives a very thorough idea to the designer about how to come up with an impressive design...c) if possible use wireframes
