[pdftex] replacing images in a pdf?

[Karl Berry]

(I realize this isn't exactly on-topic, sorry.)  Does anyone know of a
reasonable way to replace the images in an existing pdf using free tools?
(I don't insist on free software for this, though. :)

I have a pdf with unnecessarily high-res images; I can extract them with
pdfimages, and downsample them with pnmtojpeg (among others), but I
don't know any way to reintegrate them back into the pdf.  The new
versions will have the same width x height, so it's (presumably) just a
matter of getting them into the pdf in place of the old ones.

Searching the web turns up solutions involving full versions of Acrobat and
Photoshop, which I don't have (or want).

Ultimately the pdf will be included in a document typeset with (pdf)tex,
but I doubt that matters.

Just thought I'd ask.  If nothing comes to mind, life goes on :).

Thanks,
Karl

(This is one list I'm not on, so please include me with any replies.)