Are inserted images cached somehow?

[Peter S Galbraith]

If I do the following to insert an image:

  (insert-image '(image :type jpeg :file "/tmp/img_3818.jpg"))

Then I modify the image file /tmp/img_3812.jpg outside of Emacs.
Then I redo:

  (insert-image '(image :type jpeg :file "/tmp/img_3818.jpg"))

I get the same image as before inserted.  I do _not_ get the modified
version.  I can't figure out what Emacs is doing to cache the image
and therefore make it's display unmodifiable.

This is using emacs 21.2 or 21.3, and also using image.el from Emacs CVS
(which fixes a few bugs I was seeing).

Is there a way to do what I want to do?

Thanks!
Peter