iPad air wallpaper

Settings > Wallpaper > Choose A New Wallpaper should let you select an image that it's in the Photos app

Or you can do it directly in the Photos app : select the photo that you want (either open it full-screen, or in thumbnail view tap Select top right of the screen and select the image), and then use the share icon (the square with the upward pointing arrow, bottom left in full-screen, top left in thumbnail) to set it as your wallpaper.