This object is updated, how come Squid doesn't not this?

I have a CERN httpd running locally. I also have a squid cache running.
When I updated a .gif file, it turns out squid is constantly giving me the
one from the cache, not the one from the local server:

868436745.836 79 192.168.1.1 TCP_HIT/200 4115 GET
http://awt.nl/images/wierda-background.gif - NONE/- image/gif

Squid notices a TCP-HIT, but the file *is* modified. What is going wrong?
When I connect directly to httpd, I get the new file allright, so it is
squid that seems in the wrong.

The browser used was OmniWeb. A reload asked by the browser kept resulting
in the cached object, not in the changed newer version.

How do I find out what is going wrong?

Thanks,

---
Gerben_Wierda@RnA.nl (Gerben Wierda)
"If you don't know where you're going, any road will take you there"
Paraphrased in Alice in Wonderland, originally from the Talmud.
Renee: "Met veel koper maakt men hoempa." (After hearing Brahms' Festouverture)