Who are all of these photographers?

It seems likely that these are foreign journalists, and/or other individuals interested in the Potter case. We don’t really get details in the canon.

Foreign journalists

The Pottermore entry for the Prophet begins:

There is only one wizarding newspaper in Britain, discounting such small circulation publications such as The Quibbler. The Daily Prophet, whose headquarters are in Diagon Alley, is delivered by owl on a daily basis to nearly every wizarding household in Britain.

First, I feel like it would be strange if the Prophet had an international circulation but it wasn’t mentioned here. The paper is only ever mentioned as having a British readership, and is written entirely in English.

Second, there is a limit to the range owls can fly at a time. I don’t know what it is, but I’m fairly sure it isn’t global (or even far enough to get over the British Channel). The paper would need entirely new delivery methods to reach international audiences.

All of which suggests that there are other, foreign newspapers. Given that teleportation is freely available via apparition, it’s easy to send a photographer and journalist to an event happening in a different country, which is probably what’s happening here.

Although Voldemort was mostly limited to Britain, his reappearance would be of interest outside Britain. At least in part to see how we cope, and whether his antics spark copycat groups elsewhere. Oh, and the small matter that Voldemort’s return is a huge embarrassment for Fudge, the most senior magical politician in the UK. But that wouldn’t be of interest to the papers, right?

Other individuals

There are surely people writing books and biographies of Harry. The Muggle press are way more intrusive; turning up to a press conference is fairly tame. For these people, turning up to an event where he’s just defeated Voldemort (again!) could be a major scoop.

We have no canon evidence these people exist, but I think it’s likely they do.

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Competing journalists?

This is an idea that struck me right after I posted the answer above. The Pottermore entry tells us that Rita Skeeter is their “star reporter”. But at the end of Goblet of Fire, Hermione gives her a break from writing:

“I’ve told her I’ll let [Rita] out when we get back to London,” said Hermione. “I’ve put an Unbreakable Charm on the jar, you see, so she can’t transform. And I’ve told her she’s to keep her quill to herself for a whole year. See if she can’t break the habit of writing horrible lies about people.”

— Goblet of Fire, chapter 37 (The Beginning)

She seems to be taking that break seriously – when she meets Harry and Hermione in Order of the Phoenix, she talks about how other people are writing for the Prophet without her. The threat of being revealed as an unregistered Animagus is a dangerous one.

With her gone, that means there’s a vacant spot for top reporter at the Prophet. Presumably other reporters have been scrambling for that top position. This is one of the biggest stories of the year – perhaps there’s more than one Prophet photographer/journalist, trying to get the best most headline-grabbing version of this story.