<img>
<data-src>
has an original image inside,
<src>
has an encoded 1x1
gray image to for background during loading
<picture>
data-src
has an original image in different resolutions,
src
in <img>
tag has a thumbnail image for background during loading
You only need to apply class="lazyload"
to <img>
tag
background-image
(NOT recommended, please consider two first options as a better choice)