Markdown panel that is used now in Stack Exchange sites has button images with medium resolution.
It would be good to have these buttons in good quality, because in Retina display images are blurred. For example, Microsoft uses SVG images in their sites for new browsers and PNG for old.
We draw new graphical representation in vector format at http://highrobotics.com/we-did-it/appdesign/stackoverflow-markdown-buttons.aspx and it is used in our PageDown modification.
Hope that it will be helpful.