1. As <img>

    <data-src> has an original image inside,
    <src> has an encoded 1x1 gray image to for background during loading


    small random image

  2. As <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


  3. As background-image (NOT recommended, please consider two first options as a better choice)