I suggest limiting new accounts from uploading photos for 3 days, to prevent abuse.
3 days should be enough to make most people think twice before doing something so stupid, harmful and illegal. Most users don't upload photos right as they sign up anyway so this effect to legitimate use should be negligible.
I'm not saying it will prevent everything, including those with longstanding grudges, but especially if the period is not publicly announced/varies from server to server, then it will stop the impulsive trolls who can't just make a bunch of accounts.
Similar to mandatory wait laws for guns and ID creation wait period for Wiimfi community-run online service.
at that point you'll just discourage any new users if they have to gamble on whether or not their content is actually seen by anyone. account age really isn't a good indicator of anything other than soemone being dedicated enough to spam. considering this isn't the first wave of csam attacks, i can assure you that whoever is targeting lemmy with this is determined enough that account age won't deter them for long, they'll just have to slightly adjust their playbook.