Riff Raff -- Take Those Masturbation Pics Down ... Or Else

Riff Raff doesn't take too kindly to pictures of him allegedly masturbating being plastered all over the Internet ... and is now using his legal goons to make sure they disappear for good.

Riff had his lawyers fire off a cease and desist letter to TheDirty.com ... after the website posted pics of the rapper on a bed, holding his penis ... allegedly masturbating in front of a female hair stylist.

In the letter, Riff is pissed ... he says the story is B.S. and by using the pics the site is damaging his reputation. He says if the site doesn't take the snap shots down he'll sue their pants off.