The standard test applied to photographic images is "does the person being photographed have a reasonable expectation of privacy?". This test is always contextual and environmental (i.e. consider all the relevant circumstances).
Taking a photo of a child in the changing room of a public swimming pool is hugely problematic because of the vulnerability of the child, and the expectation of privacy is far more reasonable than standing in a 60,000 crowd at a sports stadium. A celebrity who earns income from being in the public eye will typically have a less reasonable expectation of privacy compared with Joanne Public (where the circumstances are otherwise the same).
Unless the legislation has changed recently, a photographer would only be criminalised if their conduct amounted to harassment or if they were engaging in upskirt / peeping Tom acts. The intent of the photographer is also relevant. Were they intending to perve, harass, intimidate or taking a photo of someone out of curiosity, fun, or attraction?
It's complex and needs to be carefully considered on a case by case basis.
Also bear in mind the ramifications of policing from a moral standpoint (morality policing) rather than a criminal standpoint (statutory policing). A person's behaviour may be unsavoury but doesn't automatically mean they should be criminalised.
