You need to modify the delivery file you use (asyncspc.php or afr.php)
Search for :
$imageTag = "$clickTag<img src='".htmlspecialchars($imageUrl, ENT_QUOTES)."' width='$width' height='$height' alt='$alt' title='$alt' border='0'$imgStatus />$clickTagEnd";
change it to :
$imageTag = "$clickTag<img src='".htmlspecialchars($imageUrl, ENT_QUOTES)."' width='$width' height='$height' loading='lazy' alt='$alt' title='$alt' border='0'$imgStatus />$clickTagEnd";