To set an image as your wallpaper first find an image you like. Click on the thumbnail so that you get the larger version of the image. Right click on the image and there will be an option to set the image as your wallpaper. Or you can also save the file locally to your hard drive. Then find where you saved the image to and then set it to your wallpaper.
Need to resize the wallpaper for a certain resolution? Click here for a quick wallpaper resizing guide.