load image

<img data-src="/path/to/image.jpg" alt="">
img {
opacity: 1;
transition: opacity 0.3s;
}

img[data-src] {
opacity: 0;
}

[].forEach.call(document.querySelectorAll('img[data-src]'), function(img) {
img.setAttribute('src', img.getAttribute('data-src'));
img.onload = function() {
img.removeAttribute('data-src');
};
});