this is actually a little different from what i’m talking about in this thread, but the last time i checked, if in the Observation Edit page in the website you uncheck a photo (instead of directly deleting that photo record), you will disassociate that photo from the observation.
this does not actually delete anything other than the association – at least not right away. for the moment, the photo record and the image files themselves remain exactly where they are. if the photo is not associated with any other observation, it will become orphaned (as described here: https://forum.inaturalist.org/t/find-your-own-orphaned-photos-uploaded-to-inaturalist/6610).
after a certain amount of time (i forget how long), the orphaned photo records will be eventually deleted. i haven’t checked to see whether the image files themselves will also be properly deleted at that time, too, when the photo record deletion happens via this process.