Justin Timberlake wants to make it clear -- the explicit picture on Mila Kunis' cell phone -- showing a penis -- is NOT J.T. -- this according to a source close to Justin.

TMZ broke the story ... Mila's cell phone was hacked, and two of the pics the hackers seized showed Justin -- one laying in bed, the other showing Justin with a pair of pink panties over his head.

There is another pic of a penis but that's it -- there's no torso, no face ... and Justin has made it clear to people close to him -- he NEVER sent such a pic to Mila or to anyone for that matter.

Mila Kunis is the latest celebrity whose cell phone has been hacked, and the hacker is leaking several pictures of a seductive Justin Timberlake ... TMZ has learned.

Four pictures have been leaked, including two which show Justin -- one, laying shirtless in a bed, and another in which J.T. is jokingly sporting a pair of pink panties over his head.  Kunis is not featured in either of the pics.

There's another photo of Mila in a bathtub, but all you see is her head.  And then there's another photo of a male which is explicit in nature, although his identity is a mystery.

There are also some texts and the hacker claims they reveal exchanges between Justin and Mila.

As we first reported, we're told Scarlett Johansson is going to the FBI today after nude photos of her that were hacked from her cell phone were published this morning.

We could not reach Kunis for comment.