In the context of porn, 'fake' usually refers to pictures of celebrities that have been doctored in some way to make it appear that the celebrity is naked. This normally involves putting the head of the celebrity (taken from a publicity photo) onto the body of someone naked (this can be any random porn picture that is a close match for skin tone etc).
It can also be achieved by 'painting' skin over the clothes of the celebrity to make it appear that they're naked.