$(function(){
$("#upload_bg_{$app_id}").css("top",($(window).height() - $('#upload_bg_{$app_id}').outerHeight())/2 + $(document).scrollTop());
});
left: ($(window).width() - $('.className').outerWidth())/2,
upload_bg_{$app_id}:要居中的div id
$(function(){
$("#upload_bg_{$app_id}").css("top",($(window).height() - $('#upload_bg_{$app_id}').outerHeight())/2 + $(document).scrollTop());
});
left: ($(window).width() - $('.className').outerWidth())/2,
upload_bg_{$app_id}:要居中的div id