"War crime" is a legal category, not a description of severity. Documentation is worth more when the person recording knows which category the material may one day serve.
War crimes are grave breaches of the Geneva Conventions and other serious violations of the laws and customs of war, committed in armed conflict: attacks directed at civilians or civilian objects, attacks expected to cause disproportionate civilian harm, torture, hostage-taking, attacks on protected objects — hospitals, schools, cultural property.
Crimes against humanity are certain acts committed as part of a widespread or systematic attack directed against a civilian population. Killing, deportation, imprisonment, torture, sexual violence, enforced disappearance and persecution among them.
Genocide requires acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group.
Aggression concerns the decision to use armed force against another state, and is a matter for the leadership of a state rather than for those carrying out the acts.
The definitions above are those of the Rome Statute of the International Criminal Court, Articles 6 to 8 bis.
Why the category matters to a person with a camera. What makes a strike a war crime is rarely the strike alone: it is the target, what the attacker knew, what precautions were taken, whether a military objective was present. That means the useful record is often of things that seem incidental — the absence of military presence, the signage on a building, the time of day, the people who were there — rather than of the crater.
Sources. Rome Statute of the International Criminal Court, https://www.icc-cpi.int/ ; Eurojust and the Genocide Network, Documenting international crimes and human rights violations, https://www.eurojust.europa.eu/publication/documenting-international-crimes-and-human-rights-violations