Similarly, how about context. If there's a photo of someone on the web and you can't find the owner/subject's contact details you could put this in a post about STDs; if that post becomes popular the subject's image is then associated with the content despite them having no relation to the subject matter and not having given their consent, which could lead to awkward scenarios for that person.

(I won't say a Facebook photo as this will be in the uploader's album, therefore associated with their account and thus can be considered non-orphaned).