I have posted some images inside questions and answers (nice feature), and missed the possibility of specifying the alignment. Actually, considering that the inserted images are never floated/inlined, I think that a much simpler solution is to change the global stylesheet so that they are always centered. I think that in most cases (if not always) that looks better than the current left alignment.
I agree that images should be centered by default, or even better, that we should be able to specify an alignment (I have no idea how difficult this is to implement, though). A workaround I've used a few times has been to input
$\hskip x in$ right before the image, on the same line; this LaTeX command simply puts in $x$ inches of horizontal space (negative values of $x$ are allowed). One then does some trial and error to find the value of $x$ that gives the alignment w.r.t. the post that is desired. I mention this workaround here, where Theo reports that seems to work on mobile browsers too.