<p>There's no way to centre an image that isn't a bit hackish, it's not functionality that is provided.</p>

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<p>If you really must, you <em>could</em> just precede the image by a load of <code>&amp;nbsp;</code> - or wrap the image in <code>&lt;pre&gt;</code> tags and use normal spaces - to manually add spacing to give the <em>appearance</em> of the image being centred (see <a href="http://meta.stackexchange.com/questions/25835/is-it-possible-to-horizontally-center-an-image-in-sofu">this</a> MSO post for examples).</p>

<p>However, this might cause problems for viewers on mobile devices and maybe tablets (not tested, but I just have a suspicion...), so I'd actually advice against this - just leave the images where they are.</p>