Anything an artist creates automatically has a sort of copyright to it.
Music works a little differently, and needs copyright based on the rhythm and such, and can only last for 10 years.
Anyone may USE a piece of art freely for self-study, but they may not take a piece of art, copy it/ redistribute it and say it's "Theirs"
They may Modify it, and (this is where it get's a little dicey) if they modify it enough, it can be arguably considered "their own work" as an Appropriation. However, that modification MUST be completely distinct, and usually involves crossing mediums/changing the subject, etc etc. This is how Parodies work and how "dada" art works or what was know as "Pop-art" (Think of the Warhol Cans)
the problem that an artist always runs into is that people like to steal things
and claim it as their own,
Unlike other professions, artist's and Musician's work are purely cultural based, and cultural based things are taught to be "Public domain," although the artifacts of the domain still have an author, and that authors work MUST be credited.