If someone on my instance checks out a post with one of your photos, it will be downloaded to the pictrs folder on my server, and then served to the user. This is how Lemmy works.
I don't see how your boost can manage to delete it from there.
Yes, the deletion would have to be federated, there is no way to guarantee anything at that point. But as I understand OP this is about acting quickly, on an upload that isn't even part of a post yet.
If I doxx myself on any service, someone can take a screenshot, it could be archived. But If I delete it before anyone sees it, there is a good chance it will never get out, if the primary platform properly removes it.
One of the golden rules of maintaining privacy was to not share what you did not want to be shared around in society. This rule has existed since cavemen learnt how to ape each others' drawings, symbols and sounds.