Yes, of course. However, it is all about how you distribute, by any means, the copied work.
To put it simply, you become a thief when you represent copied material as your own, sell it without authorization as a copy or otherwise diminish the value of the original work.
This is a complex legal area, but if you copy a Manet for your own pleasure I doubt that the art police will come knocking at your door. However, if you get very good at it, some would become very interested in your work, legally.
Copy away, but don't misrepresent it or sell it without permission.
First of all, the question is begging to be asked: Would you want it done to you? As a muralist, I can tell you that it is not okay to recreate another's work without the expressed permission of the living or deceased artist and his chain of command. AND the art police do exist. A colleage of mine painted Disney characters in the church basement of a pre-school center. The organization caught wind of it and marched him down to prime the work out, all this after he received legal threats from them. Come to find out one of the kid's parents worked for a sub-contracted organization of Disney and innocently, happened to talk about how great it looked to her fellow workers. Don't fool yourselves. If you want to copy a work as a practice method, go for it. Do not let it outside your studio, claim it as your own or sign it.
Yes and no. It is okay to copy anothers' work for purely instructional purpose (with you identifying the artist whom you copied). Other than that, copying is only okay with written permission from the artist. This also goes for characters, etc of the artist's creation. Fan art must be accompanied with whom the character is copyrighted to.
Ripped art has become even more serious of late with scanners and the like. They will hunt you down if you illegally copy anothers' work. (By the way, any art posted on the internet is legally copyrighted by the artist. Further details are available at DeviantArt.com's FAQ section.) Even a mural painted for free by a teacher at my elementary school of a few childrens' book characters had to be painted over. Be careful. Copyrights matter.
Advice to any artist: Mark on any website that your art is not to be copied. Digitally watermark anything. Be sure to know you have proof your work is yours. There are many helpful articles in the DA forumn. Join groups against ripped art. Don't rip art yourself.