According to Twitter, “This policy update will help curb the misuse of media to harass, intimidate, and reveal the identities of private individuals, which disproportionately impacts women, activists, dissidents, and members of minority communities.” The company notes that photos taken of people from public events will generally not violate this policy, so basically this will cover photos taken of people privately.
https://twitter.com/TwitterSafety/status/1465683093436739588
It does not seem that Twitter will be actively seeking out photos posted on its platform to check whether or not it violates their guidelines, and will instead rely on first-person reports and then review it, meaning that even if an image is reported, there is a chance it might not necessarily be taken down.
The company notes, “We will take into consideration whether the image is publicly available and/or is being covered by journalists—or if a particular image and the accompanying Tweet text adds value to the public discourse—is being shared in public interest or is relevant to the community.”