I'd like to hire artists to create some assets for an open source game project. I'd like all the assets I use to also be released under a compatible open source/free content license (e.g. Creative Commons), so that the entire project is open source. I'm happy to pay for the work to be created and released publicly.
What's the best/correct way to go about this? Some potential artists I've spoken to aren't familiar with open source licensing, which is understandable as they're used to working for private proprietary projects. Should just I ask them to agree to release the work under the terms I choose? Or ask that they assign the copyright to me, and then I release it as I see fit? I don't have any experience hiring someone so I don't know what the "done thing" is in this situation.
What's the right way to show proof that the assets were released under the stated license and/or assigned to me? As in, should I use a contract, or is a simple agreement via email sufficient?
Thanks in advance - I hope this isn't too vague a question!