I agree with @ddude1116 about the definition of plagiarism. Collages are, by definition, compilations of a variety of other people’s work. There could be a case made for receiving permission to use it or clearly acknowledging the original artists, though. Along the same lines as referencing materials used for writing papers, articles, books, etc.
@Jeruba I would say that is a piece of found or recycled art. The bike was made for practical use and Picasso repurposed a few parts to make a sculpture. If I rip the upholstery off of my chair and make a wall hanging out of it then sell it in a yard sale, I wouldn’t feel compelled to share my $3 with the company that made the chair. Of course, I’m not Picasso and my wall hanging could never compare to “Bull’s Head,” but the use of materials is a fairly legitimate comparison.