Since we can only use Markdown and a limited subset of HTML/CSS is it possible to center an image, or any block element for that matter, on the Stack Exchange sites?
Random 470×320 px image:
You can add artificial margins directly into the image, if you don't mind, that they will be preserver proportionally, so the graphics itself will be very small on small displays.
Modified image, canvas extended to 1600×320:
(This is suboptimal solution, centering feature wanted.)