Ask YC & PG: Can I use Artwork on my websites?

I was wondering how can I use images of paintings by Picasso, Bruegel, Robert Rauschenberg and etc on my website? Can I display them on my website? I mean I've seen such images on Wikipedia and even on http://www.bugbear.com. Can I use them on my website? If no, then how do I go about acquiring some images? What is the correct and legitimate way to use images of an artwork? Please enlighten me.

Thanks in advance.