Add background image using Javascript CSS property

You need to add the actual contents of the variable weatherImage, right now you're just setting the url to the name of the variable. Try changing your code to:

$('body').css('background-image', 'url(' + weatherImage + ')');

It is a little hard to tell what is not working since you don't have included the entire script and you also don't show how and where you call selectImage(). The following should work though:

// change api key to whatever you are using
var OpenWeatherKey = 'your-api-key';
var locationUrl =  'http://freegeoip.net/json/';
var weatherImage = "";
var imagePhoto = {
   thunder: "http://www.tabinotebook.com/wp-content/uploads/2017/02/jeremy-bishop-72584.jpg",
   rainy: "http://www.tabinotebook.com/wp-content/uploads/2017/02/lukas-budimaier-131299.jpg",
   cloudy: "http://www.tabinotebook.com/wp-content/uploads/2017/02/odair-faleco-192489.jpg",
   snow: "http://www.tabinotebook.com/wp-content/uploads/2017/02/james-padolsey-154227.jpg",
   sunny: "http://www.tabinotebook.com/wp-content/uploads/2017/02/tomas-salas-81161.jpg"
};

function selectImage (id) {
   if(id >= 200 && id <= 232) {
       weatherImage = imagePhoto.thunder;
   }
   else if (id >= 300 && id <= 531) {
       weatherImage = imagePhoto.rainy;
   }       
   else if (id >= 600 && id <= 622) {
       weatherImage = imagePhoto.snow;
   }   
   else if (id >= 801 && id <= 804) {
      weatherImage = imagePhoto.cloudy;
   }
   else if (id === 800) {
      weatherImage = imagePhoto.sunny;
   }
   else { 
      weatherImage = imagePhoto.cloudy;
   }
   // we set the background first after the weatherImage has been assigned a value
   // above
   $('body').css('background-image','url(' + weatherImage + ')');
}

function getLocation(){
   $.ajax({
       url : locationUrl,
       dataType: "json",
       success : function(data){
           var country = data['country_name'];
           var city = data['city'];
           var latitude = data['latitude'];
           var longitude = data['longitude'];
           $('#location').html(city + ', ' + country);
           var Weather = "http://api.openweathermap.org/data/2.5/weather?lat=" + latitude + "&lon=" + longitude +"&APPID=" +OpenWeatherKey;
           getWeather(Weather);
        }
    });
}

function getWeather(url){
   $.ajax({
       url:url,
       dataType:"json",
       success: function(data) {
          var weather = data['weather'][0]['main'];
          var temp = data['main']['temp'];
          var icon = data['weather'][0]['icon'];
          var id = data['weather'][0]['id'];
          document.getElementById('icon').src= "http://openweathermap.org/img/w/" + icon + ".png";
          $('#weather').html(weather);
          $('#temp').html(temp);
          // Change F to C, C to F 
          var fahrenheit = Math.floor((temp) * 9/5 - 459.67) + '\xB0F';
          var celsius = Math.floor((temp)- 273.15) + '\xB0C';
          var $tempDisplay = $("#temp");
          $tempDisplay.html(celsius);
          $("#button-f").on('click', function() {
             $tempDisplay.html(fahrenheit);
          });
          $("#button-c").on('click', function() {
             $tempDisplay.html(celsius);
          });

          // select background image based on id
          selectImage(id);
       }
   });
};

getLocation();