Welcome to the board, but I have to be completely honest and utterly blunt.
High traffic means jack s**t, however it seems from both posts that's your "big draw" for the photographer to submit. You're asking for photographers to submit for free with photos they could get hundreds, and sometimes thousands, of dollars if the content is right from other sources. Photo credit on a high traffic site means nothing, NOBODY is reading it and/or could care less about it except the photographer, their family, and other photographers. I've been published (paid) on some of the most visited sites in the world, as well as the most circulated magazines/newspapers in the world, and not once has anyone but those already listed above mentioned the photo credit.
While on the subject, it's a startup but yet is high traffic? I doubt that.
Copyright issues, only if you try to take the copyright of anyone who is dumb enough to submit free. If you try to rob their copyright and haven't given them a darn thing I find it unethical - yet companies do it all the time - and if not clear you will run into problems. Some people are gullible enough to think photo credit matters and submit before reading, that's their own stupidity driven by ego, but the site will get a bad name. If stealing images from the web, expect a lawsuit or a hefty bill for the illegal activity. If you're talking about copyright about the person in the photo (the celebrity) then you need a lesson on commercial vs editorial usage before starting up any site.
Sorry to be so blunt, but beating around the bush does nothing.
So the answer is simple, PAY FOR THEM, just like TMZ and other sites do. Non-profit celebrity site seems like a strange concept, gossip site doing it for free?? Don't see a charity angle so you have really no right asking for people to work for free for your own benefit. If only labor laws covered creative work to.