Since a long time I was wondering how to replace the Mona Lisa's face with my own one. The main problem was how to to make my skin the same as that of the Mona Lisa, and finally I did it thanks to the help of my friend Emanuela. This is a professional photomontage technique, and my personal image (the Mona Crupi) is just an example. The same method can be applied to other images.

Before beginning I want to apologize to Leonardo da Vinci, who at this moment will be turning in his grave.
In order to achieve the target of this tutorial, you can follow these steps:

1) Open the your image file and the one with the Mona Lisa. In the latter one, duplicate the layer with the Mona Lisa (which will be used at the end for some last-minute retouching with the stamp tool).

2) On your image, select the brush tool and set its hardness to zero. Now select the quick mask tool (the rectangle with a circle inside), double click on the quick mask and a dialog window will open, here set "Selected areas", and now use the brush to color over your face.

3) Remove the quick masck by clicking on the rectangle near to quick mask one, the brush color will disappear and we will see a selection on our face. Copy and paste this selection over the Mona Lisa's face.

4) After that you have to change the dimensions of the pasted selection with your face, by choosing Edit - Free transform. And then you can adjust the color (go to Color balance), the brightness and the contrast in order to mimic the original brightness and color of the Mona Lisa's face.

5) Now comes the most annoying and difficult part to explain. Select the Clone stamp tool and go to the duplicted layer with the Mona Lisa's image.

Take a well defined point on this layer by pressing ALT and clicking over this point with the clone stamp tool. As a reference, I used the line visible on the Mona Lisa's forehead. Now go to the layer containing your face and click on the same point you choosen on the duplicated layer. In few words, you have to color the parts of your face by using parts of the Mona Lisa's face and the clone stamp tool. Adjust the opacity to 40-50% where you don't want to copy parts of the Mona Lisa's face over your own one, this will give the impression that our eye, our eyes, our nose and mouth are part of the painting.