Using a Background Image Loader

You're attempting to check if a background image has loaded by setting a background image on the body tag and then using the load() function. However, this approach doesn't work effectively for background images.

To truly ensure that the background image has completely loaded, you can utilize a different technique:

$('<img/>').attr('src', 'http://picture.de/image.png').on('load', function() {
   $(this).remove(); // prevent memory leaks
   $('body').css('background-image', 'url(http://picture.de/image.png)');
});

This method creates a new image in memory and attaches a load event listener to it. Once the image has loaded, the load event is triggered, and the background image is set on the body tag.

In vanilla JavaScript, this can be implemented as:

var src = 'http://picture.de/image.png';
var image = new Image();
image.addEventListener('load', function() {
   body.style.backgroundImage = 'url('   src   ')';
});
image.src = src;

You can also create an abstracted function to handle the image loading and return a promise:

function load(src) {
    return new Promise((resolve, reject) => {
        const image = new Image();
        image.addEventListener('load', resolve);
        image.addEventListener('error', reject);
        image.src = src;
    });
}

const image = 'http://placekitten.com/200/300';
load(image).then(() => {
   body.style.backgroundImage = `url(${image})`;
});

This approach allows you to reliably check for the completion of the background image loading and ensures that code is only executed once the image is fully loaded.