Material that cannot be shown to be what it claims to be is material that will not be used. The protocols call this chain of custody, and for digital material it starts on the device.
Do not edit the originals. No crops, no filters, no rotation, no compression, no re-saving. Work on copies and keep the originals untouched, including their metadata.
Record a hash. A checksum of each file, written down at the time, demonstrates that the file has not changed since. This is the single most useful technical step an untrained documenter can take.
Keep a log. Who recorded it, when, where, with what device. Who has held the material since, and when it moved. Every transfer, in writing.
Back it up, in more than one place, encrypted. Devices are seized, lost, broken and searched.
Do not publish before you have thought about it. Publication can warn a perpetrator, endanger a witness, and — where images are re-shared and re-compressed — degrade the material's value as evidence. Some material should go to an authority or an established organisation rather than onto a social network.
Where to send it. In Ukraine, the Office of the Prosecutor General maintains the mechanism for reporting war crimes, https://www.gp.gov.ua/ , and the national police receive reports at https://www.npu.gov.ua/ . Internationally, the Office of the Prosecutor of the International Criminal Court accepts information under Article 15 of the Rome Statute, https://www.icc-cpi.int/ . Israel's investigations of incidents involving its own forces are conducted by the Military Advocate General and the fact-finding assessment mechanism; material relating to them is submitted through those bodies.
Sources. OHCHR, Berkeley Protocol on Digital Open Source Investigations, chapters on preservation and chain of custody; 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
This is not legal advice. Whether material is admissible anywhere is a question for a prosecutor and a court in the jurisdiction concerned.