The answer is: It depends. First off, you have to find out if the thing you are recreating is trademarked (like Mickey Mouse) or not. Most fiction like mine is copyrighted, but the characters are not trademarked.
The second question is this: Are you creating your own work based on the original work, or are you simply copy and pasting? For instance, if you're creating a wallpaper are you copying copyrighted images directly, or are you drawing your interpretation of ideas, themes, and people in the books? The former infringes, the second is in a large gray area.
You see, US Copyright laws protect 'Derivative works.' One common form of the derivative work is a collage. Artists have been upheld in their right to take a bunch of clippings from a magazine, cut them apart, make a collage of pieces, then sell that as a new piece of artwork. Many people argue that drawing a new work based on your concept of a character is a derivative work, and is protected.
Question three: How similar IS your artwork? If it's inspired by the book, and you're not using any actual copyrighted images, you're in a probable safe zone. For instance, people who sell T-Shirts with things that look a lot like heart containers from the Zelda games. Link himself is trademarked, but a heart...a heart can't be trademarked. Who's to say those heart containers are even from the Zelda games? The people selling the T-Shirts can hide behind the fact that the shirts don't HAVE to be linked to the games. (Even though everyone buying them knows exactly what they relate to.)
Question Four: (and this one is probably the most important) What are your intentions? As has been mentioned, if you're not seeking to make any money off of the artwork and you're not at risk of diluting or defaming the actual copyrighted work, then it's likely nobody will care. It would be stupid of companies to chase down people who make wallpapers based on their properties, since those wallpapers act as free publicity, and the people making them aren't trying to earn money off of them. If you were selling posters of that image, you might draw attention--but, then, for reasons argued above you could have a strong argument on your side. (Assuming you wanted to pay the legal fees to fight the person trying to stop you.)
Question five: What does the author/creator think? Some people fight fanfiction (and to a lesser extent) fanart because they feel it dilutes their property too much. Me, I'm flattered, and have no object to fanart. Fanfiction is more sticky, since authors risk getting sued over their own books with fanfiction. I've written an explanation on the Warbreaker rights page.