Kevin Federline -- That Penis Pic Is Bogus ... My Real Dong's Hella Bigger

Kevin Federline is calling BS on the naked pic supposedly featuring his legendary baby maker -- telling people the shot is faker than his rap career.

The shot's been all over the Internet ... apparently showing K-Fed gripping his Sexcalibur like he's preparing for battle.

But sources close to Federleezy say he's 100% certain it's a Photoshopped pic ... not only because he has tattoos down by the dragon's den which aren't in the pic -- but he also claims his shlong is way shlonger in real life.

K-Fed's penis couldn't be reached for comment.