If the wallpaper you have chosen is not high resolution, it will look grainy. To make the right choice, make sure you search for one based on the specifications of your monitor. You monitor resolution may be 800 x 600 or 1920 x 1080, for instance. The right wallpaper will have the right resolution and aspect ratio.
You can download beautiful wallpapers from tons of sources on the web. For personal, non-commercial use, you can use any wallpaper that appears in the Google search results provided it doesn’t reveal anyone’s personal information.