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.
This was already in my code, but I commented out, maybe I should just let it be in there real quick before opera changes to webkit: https://github.com/mrcoles/markdown-css/blob/016bf0863867aed...
If not opera, lmk what browser you're using.