把它们加起来：

The gain from declaring a width value
and an height value (which may not be
the original physical dimensions of
the image) or from css declarations
(like width: [valueX]; height:
[valueY];) is that it helps speed up
the rendering of the page. The browser
knows how much space to allocate a
particular area of the page: it will
even draw image placeholders on a
first draw,a first parsing+rendering
of the page. When one does not define
any width and height,then the browser
has to download the image and then
figure out its dimensions,space to
allocate and then redraw the page.

这似乎是我看来最有利的效果.