I've been mostly oblivious to this aspect of MMD but found it in the Byword documentation.
Multimarkdown supports things like width and class attributes for images, if they are included as link references. Handy.
I really should read more documentation.
UPDATE: Here's an image reference for one of my inserted images.
[fakemacro]: http://www.macdrifter.com/uploads/2011/12/Screen%20Shot%2020111223_232543.jpg style="display: block; margin-left: auto; margin-right: auto; width: 600;"