Not sure how experienced you are in this (the beauty of an open forum lol), but I'm going to iterate, and my experience has been the opposite of this.
OP is asking about whether Instagram will delete photos.
First, copying images from other IG profiles is not the idea of IG. I received a cease and desist letter last week for doing so. This wasn't from IG, this was from the original content poster under another username on IG. However, again, this is not the question at hand. The OP wants to know about IG deleting photos. The photos remained after this, IG didn't delete anything or get involved even though the images were likely reported several times, but to avoid legal battles, we removed the images ourselves.
If you're copying random image from randoms on other websites, same risk, but you could do it anonymously to avoid C&Ds.
However, to address your concern on whether IG will delete images or not, generally they don't care about other people's content from other websites. For example, ripping YT content to IG and vice versa. Google and Facebook don't really give a fuck about each other. They don't like each other, and will not limit content on their own platform so users leave to go to the consequent platform.
Confusing maybe, but that's my experience. Not a lawyer.
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