From an email discussion between Josh and Rudi:

Hey Josh

There's no standard or guideline of which I'm aware. Books on design, 
layout, and typesetting might be able to provide something if someone's 
motivated to do the research, or a UX designer like Andy might be able 
to point you straight to something useful. What follows is my personal 
opinion only.

If images are left-aligned in LTR text, we would definitely want them to 
be right-aligned in RTL text (which Publican does by default; I just 
verified this with Publican 3.1 in Hebrew) or the output becomes very 
weird indeed.

I would argue against centring images on two grounds:

1. We encode information in the level of indent in our docs. Elements 
that share the same level of left margin share the same level in the 
hierarchy of information that we present. A centred image with an 
effectively random amount of left margin breaks with this model.

2. Images are not necessarily all the same width. Even if we ignore 
point 1 completely, a single centred image, or a number of centred 
images of the same width, doesn't look so bad. If a page contains 
multiple images of different widths, ragged right presents much better 
than centred images of different widths, especially when it's already 
surrounded by text that's set ragged right. If we do consider point 1, 
then the breakage is much, much worse.

To my mind, the question of "why don't we centre images?" really isn't 
very different from "why don't we centre body text?"

Cheers,

Rudi