[racket] confused beginner

Hi, I've got a really simple problem, I think.  Why is it that when I try
to do the following:

(require 2htdp/image)
(define path-stringy "images/some.png")
(bitmap (path-stringy))

I get the following error:
 bitmap: expected the argument to specify a local path (via a string) or a
module path (e.g. `icons/b-run.png') in: (bitmap (path-stringy))

I mean, shouldn't path-stringy evaluate to a string before it's passed into
bitmap?  If not, then how do I use bitmap for anything dynamic?

Thanks,
Andrew
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