Oh, interesting. Chrome does play nice with that.

Also, it looks like maybe you have to make the img content "", in order for the :before to work—otherwise it doesn't show up, at least in chrome, and with that demo it appears as nothing at all in firefox. It's a cruel world…

In the bookmarklet (included at top of demo) I do an extra step to make images inside links smaller and appear inside the link markdown—not what you were getting at, but it is a little something something.