禁止浏览器的图片缓存。
通用内存用于缓存图片数据，除非通用内存不足。在内存不足时，每次都会重新生成图像数据。
General memory is used to cache the image data, unless it is in short supply, in which case the image data is regenerated each time.
这种方法还适用于你需要从多个服务器取得许多不能被缓存的图片。
This trick can also be immensely effective when you include lots and lots of smaller images from several servers (like gravatars for example) as these are not likely to be cached.
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