The way I usually work is that I give a detailed description of what I want. I then negotiate a flat price for the work to be done. If I ask for a change that was not part of what was described in what I originally specified, then I have to pay extra at a pre-arranged hourly rate.
Any art work done for me will be owned solely by me, but may be used in your portfolio or your website freely to show what you can do. If you want a special credit for you work listed whenever it is used, that may be able to be arranged too. I anticipate a need coming up for some work soon. Anybody interested or know someone who might be? Have them email me at firstname.lastname@example.org with a portfolio and how you want to charge. Amateurs are welcome and I don't care what your resume says. As long as you show me in some way that you can do it, that's good enough. Even if you don't have a portfolio, tell me what you can do and I'll consider you.