[racket] image.ss: can one make a rotation-invariant middle?

In playing around with image.ss, I've run into the following problem: the
center of an image does not appear to be rotation invariant.  Thus, if you
use place-image/align in order to place a triangle, say, that you then
rotate continuously, you will see the triangle wobble as its effective
center moves around.

Is there any way around this?  I tried using pinholes, but pinholes seem to
require that all images involved have pinholes, which is not quite what I'm
going for.  What I really want to do is to declare a stable center for an
image, but I can't quite figure out how to do that.

y
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