A) Do not use other peoples art work to demonstrate what you want, this encourages plagiarism.
C) Descriptions:
.....I) Describe what colours you want, and what resolution (size) you want the image(s) in.
.....II) Avoid words like 'cool' or 'detailed', as they don't tell the artist anything.
.....III) Works like 'rough' and 'smooth' are useful.
.....IV) Styles like 'vector' or 'painted' help alot as well.