The pages don't fit an 800 x 600 pixel monitor, and they're still common. There's no good reason not to make your design fluid enough to fit different monitor and browser window sizes.
Your first page is useless. It's just a big graphic without any actual text, which means that search engines can't and won't index it; it contains no content so people will be forced to let it load and then click on it to get to the real content. It also doesn't obviously lead one to click on it, as it isn't obvious that it's not just a place holder for a site yet to come, and gives no real clue as to what the site is. People make a decision to stay and use a website or leave and go somewhere else within moments of hitting a page. Here you are giving them too many chances not to stay.
Then once you go to the next page you've got the problem of what the site is again. I had to stop and try reading the text in order to attempt to figure out what the site was for, and then I still wasn't really sure. People don't read web pages, they look for something they're interested in or a link that looks like it will take them there. As such copy needs to be short and to-the-point. Adding a short (six to eight word maximum) tag line under the site logo that tells what the site is will allow people to understand what it is immediately. You can leave off the label "Home Page" on the page as it adds no content. In the navigation change the link to simply "Home" as that's what people expect.
In the various sections it's poor design if links aren't obvious. You shouldn't have to include instructions of "Clic on picture" (which should read click and not clic).
After clicking on a picture I got to a page with a lot of images. I decided to explore other sections, so I clicked on the "back" button (one of the most used interface items) and it didn't work. I hadn't expected, nor did I notice, that you opened up a new window for the page. Why? I'm still on the site, aren't I?
The site is bilingual, yet the guest book (called "gold book" in English for some unexplained reason) has headings in six languages. Shouldn't there be just two as that's what the site is in which assumes your visitors will know one of those two or probably won't be using it?
If I'm on the French home page and prefer English, I can click on the flag icon and get the same page in English. However if I decide I'd rather have the page in French and click on the French flag, I get kicked out to the useless "introductory" page. I would think that changing languages would yield the same page in the other language rather than sending me someplace I didn't expect. This is especially problematic if I came to an English page of your site from a search engine and wanted to change languages and suddenly had a page with no content. I might not realize, more so than before, that the page was just one giant link to get to the content.
You specify only one font for your text (font-family:MS Sans Serif) which means if someone doesn't have a font with that name, they get a default font for their browser. In my case I get Times, which is far from any sans-serif font. Had you specified font-family:MS Sans Serif, sans-serif I would have had something a lot closer.
Although not a big problem as most people will use one language or the other, but it would probably be better if your navigation links were in the same order in both menus. There also is one item (links) missing from the English menu.
Your contact page has this code:
<IMG id=(12) height=15 src="ico~script" width=20 name=script>
which shows up as a missing image on the page.
you also suggest that if someone wants to simply leave a comment that they should use the gold book. "Gold book" should be a link to that page so they don't have to look for it. First name and name should read either First and last name or simply name (do you really care if they include their full name or not?). The space for the message is the same size as that for their name and email address. I guess you don't expect them to send more than a few words? I would give them at least five or six lines in the text input box to give someone enough space to finish a thought.
I would also title each page with the specific section, i.e. Savoie Nature: Flore so that if some one bookmarks a page the title saved with it is more descriptive. It may also help with search engines figuring out how to rank the various pages.