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Don’t imessage and whatsapp automatically store all images received in the iphone’s photo library?

iMessage no, WA by default yes, but can be disabled.

So no need of hash collisions then. One can simply directly send the child porn images to that person via WhatsApp and send her to jail.

But then you're searching and finding child porn images to send.

I'd be surprised if there won't be a darknet service that does exactly that (send CSAM via $POPULAR_MESSENGER) the moment Apple activates scanning.