Hilary Duff -- Contacts FBI Over New Pics ... That's Not My Naked Body!!

Hilary Duff is going on the attack over explicit new Internet photos that purportedly show her nude -- because she says the woman in the pics is NOT her.

A new batch of extremely graphic photos surfaced Friday ... but Duff's rep says they've examined the alleged pics of Hilary and determined they are fake ... and they've alerted the FBI.

We're told ... Hilary has very "defining birthmarks" and recognizable tattoos ... that do not appear on the woman in the new photos. One of the tattoos is an anchor on the inside of Hilary's left ankle that she's had for years ... but again, it doesn't show up in any of the pics.

We're told they've forwarded the links to the FBI ... and anyone who publicly states the pics are of Hilary is "going to meet legal recourse."