There's another <meta> tag you might try too:
<META HTTP-EQUIV="expires" CONTENT="0">
It may or may not work, depending on the browser?
It tells the browser to check if there is a newer version available than the one in the cache, and then is supposed to load the new one.

I just looked at your page again and found that it was scrambled. What happened was that the main picture was moved to the first menu button.
A simple refresh/F5 fixed the problem, and that is really the preferred/common way to fix such problems.
You may tell Xara to give the main image a specific name, by adding a name to it, like 'filename="main_image"'
Then it will not be renumbered and mixed up with the other exported images even if you rearrange/edit your site.
BTW, I have never had such problems when changing things. Every time I've uploaded some changes of course I need to F5/refresh to see the changes.
I have never gotten any images moving around into buttons etc, even though I usually don't add filenames to my images, but let Xara choose a number.
And when visitors come to the page for the first time they will see it correctly anyway. I think most browsers by default flushes their cache when closed?
Using those meta tags may cause all your pages to reload every time, even when going back and forth between them. That may be more of a nuisance than appearing scrambled now and then (after an update).