Often just nuking all image uploads made during a certain time period. Which is why old image threads in Lemmy have time periods littered with broken images.

Yes

Often they delete all images during the time frame of a CSAM attack, as that has been the only real feasible way to ensure images weren't left behind. Though I think a few images have started using AI detection methods to remove images like that automatically (read up on that here and here), also Pict-rs now has a Log linking uploaded images to the user, so now images can be purged with the users.

Admins can purge posts manually which actually deletes them. Or use tools like db0's lemmy-safety that tries to automatically search for CSAM and wipe it.

I think the problem here is the user didn't finish their post which means the photo was uploaded but not associated with a post and therefore not purgeable that way.