The "great artists steal" line comes not from Picasso but from T.S. Eliot. Lots of previous discussion about this:

https://hn.algolia.com/?q=picasso+eliot#!/comment/forever/0/... eliot

The line that elliots (not to be confused with T.S. Eliot!) quoted turns out to be authentic, though—or at least there's some evidence for it:

http://books.google.com/books?id=OJTKZeXaUvkC&lpg=PA53&dq=%2...