When you use CSS, you can center graphics with DITA2Go macros. For example, to center all images:
[GraphStartMacros]
* = <div class="img">
See §32.4.2 Replacing or surrounding a graphic with macro code.
div.img {text-align: center; }
Or, if DITA2Go maintains CSS for your project (see §31.8.3 Overriding styles in DITA2Go-generated CSS files):
[CSSEndMacro]
div.img {text-align: center; }
You have to use <div> because CSS applies text-align only to block elements, and <img> is not a block element.