You need a file that's the pixel resolution of your monitor. An example, the resolution on my laptop screen is 1600 x 900 pixels. Look at the pixels not the inches. And make sure whatever you grab from your google search isn't copyright protected. Just because it pops up in the search does not make it OK to download it.
deviantart is a good place to look for things like this, many people put a creative commons license on their images there so it's OK to download them. More and more people on Flickr are adding a creative commons as well. But when in doubt, ask the artist.