Painting Movie Stars from Photos?

I often sketch movie stars for the fun of it. I use photos I see on the internet. I use watercolor or charcoal and of course it is by no means an exact copy. I do it for my own pleasure, but now I am encouraged to show my work at a gallery. Is doing a recognizable likeness of Angelina Jolie, Hally Berry, or Noomi Rapace and the like an infringement on their rights or the original photographers copyright? I would think so, but if it is so then why do I see the same photos on dozens (perhaps hundreds) of sites? Surely all those people are not getting permission from the stars.