<label class="radio"><img id="boyImg" class="radio-img btn-boy" src="/wp-content/uploads/2016/09/boy.png"><input type="radio" name="Gender" value="Boy"></label>
<label class="radio"><img id="girlImg" class="radio-img btn-girl" src="/wp-content/uploads/2016/09/girl.png"><input type="radio" name="Gender" value="Girl"></label>

$('input[type=radio]').on('change', function() {
    $(this).closest("form").submit();
});

$('input[type=radio]').on('change', function() {
   if($('input[value="Boy"]').is(':checked') || $('input[value="Girl"]').is(':checked')){
        $(this).closest("form").submit();
   }
});