First of all, you can't mimick watchersweb.com, because a WHOIS reveals they are located outside the US. I assume (am I right?) you are in the US, so you are subject to our legal system, not the one in New South Wales, Australia.
When you receive a submission, you should ask for documentation as well. If they mail submissions in, they can mail in a photocopy of their driver's license (or other acceptable photo ID) and one additional supplementary ID (which need not be a photo ID, though you'll be more comfortable if it is). BTW, make sure to tell them that their face needs to match the one in the photo, so that both the photo and the ID should clearly show the person's face, to establish identity.)
Oh, yeah, and DO check the ID items to make sure that the submitter actually is 18 or older.
If they are sending you submissions electronicall (as e-mail attachments, for example), presumably if they can scan a photo, they can scan ID items as well. Why not ask for what will make you perfectly safe?
Then, there's the matter of a release. You should put a basic one on your site and require that it be filled out and submitted along with the photo and ID items. Feel free to "borrow" this release and discuss it with your attorney, who knows what local laws prevail:
http://www.girlsdotcom.com/nude/release.htm
If you can't do this right, don't do it at all. It's way too risky.
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Producer of truly original teen/young woman-oriented adult content at
Wonders of the Unseen World