Why don't :before and :after pseudo elements work with `img` elements? [duplicate]

Question

This question already has an answer here:

• Does :before not work on img elements? 11 answers
• CSS :after not adding content to certain elements 3 answers

I am trying to use a :before pseudo element with an img element.

Consider this HTML and CSS...

HTML

<img src="http://0.gravatar.com/avatar/this-is-not-a-hash" alt="" />

CSS

img:before {
  content: "hello";
}

jsFiddle.

This does not produce the desired effect (tested in Chrome 13 and Firefox 6). However, it works with a div or span element.

Why not?

Is there a way to make pseudo elements work with img elements?