We are using the excellent Lazy Load XT jQuery plugin in this example. In order to make it take effect, we need to replace the srcset attribute with data-srcset. This is one situation where the rwp_attributes() function is handy.

$attribute = rwp_attributes( get_post_thumbnail_id(), array(
  'element' => 'img'
) );
echo '<img data-srcset="'.$attribute['srcset'].'" sizes="'.$attribute['sizes'].'">';

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