In this tutorial is explained how to make pop art pictures and how to make same pictures like Andy Warhol.
Find your pictures and open it in photoshop.

Shift+ctrl+U to desaturate your picture
(image/adjustment/desaturate)
filter/artistic/cutout. In cutout menu try to make best effect by changing parameters, and then click OK. (If you want to make same effect like on Warhol's pictures instead cutout you can use treshold adjustment, or try to combine both)

make new layer ctrl+shift+n (layer/new layer), and name it ‘color’
Select t-shirt, save selection (select/save selection), and name selection ‘t-shirt’
ctrl+shift to deselect
now select skin without mouth and eyes, name selection ‘skin’
select mouth, and name selection ‘mouth’
select eyes (‘eyes’)
name selection of background-‘back’

Now go to layer ‘color’, then click in menu select/ load selection and load ‘t-shirt’, color them, do same whit other selection, but color them with different colors.

In your layers menu, click on ‘normal’ and change it to ‘multiply’

Right click on top of your document where is name of your file and choose duplicate, and repeat that 2 times. In new files change colors of t-shirt, eyes, mouth, skin and back. Flatte all files (layer/flatten image). With move tool (v) place your pictures in one document (if you have to scale layer to place it corectly go to edit/transform/select), and that's it.