headers (?): Firefox doesn't revalidate the thumbnail

[Accipiter]

Hi all,

I have no idea if this is the right forum, but none seemed to match. But since it is with php that I will send the headers, I will ask here nontheless.

I am creating an image gallery site.
When I upload an image, it automatically creates a thumbnail from specified coordinates on the image. When I change these coordinates, the old thumb is deleted and a new is created.

Problem is that, after submitting the new coordinates Firefox (1.5 atleast) doesn't revalidate the thumbnail preview image but uses an old cached one. (IE actually works fine).
Though, if I press submit on the page where I edit the image information (like description), then it revalidates the image. But, both the Thumbnail edit and the Image information Edit page has the same URL. Why is this?

Headers you say.
Well, yes. This in an example of the headers that I send with the thumbnail:

[tt]Content-Location: thumb.php
Vary: negotiate
TCN: choice
Accept-Ranges: bytes
X-Powered-By: PHP/4.3.1
Content-Length: 6161
Cache-Control: public no-cache must-revalidate
Pragma:
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Content-Disposition: inline; filename=big3_thumb.jpg
ETag: "5_1169038031"
Content-Type: image/jpeg
[/tt]

If you wish to help, I just set up an account on the site and uploaded an image which you can try it out with.
Just log on and I have provided a link to the image on the index page.
URL: [url=http://www.jirenius.com]http://www.jirenius.com[/url]
Username: [b]phpfreaks[/b]
Password: [b]phpfreaks[/b]

Any advice is appreciated.

/Samuel Jirenius