I like it. Simple and has a touch of elegance to it. Let me be the first to say that, in my opinion, you have done a superb job. Here're a few things I noticed, take them for what they're worth (which isn't usually much.

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As for load times, on my DSL the sidebar images are loading perceptibly slow. I look at the file size of the sidebar images and they're not that taxing -- 120kbs -- but I can only imagine what they'd do on dial-up.
- Some of the randomized sidebar images conflict with the "Plan Design Build Remodel" words and obscure them.
Here's an example. But it looks as though it renders properly on all the major browsers.
- You may want to handle photo 0 as it brings up a Red X and tells me that there are no more photos in the series. However by doing this it breaks the layout:
here's an example. The problem is that the "Previous Photo" link is always displaying, even if I'm at the last photo in the series and this causes errors, because from photo 0 I can go to Photo -1, and keep going. This also happens when I'm at the last photo in the series, despite being told so.
Here's an example. (only in this case, the "Next Photo" is always displaying)
- I'd really think a link back to the gallery main page should be displayed on the "Previous/Next" photo browser. There should always be an escape route, because I like to use tabbed browsing and sometimes I want to start a fresh history with a newly opened tab and not mess with my previous tab's back/forward state.
- Some of the photos are only half displaying.
Here's an example. Or is it supposed to show the gray?
- I think you should choose a bit more a browser-friendly resolution for some of the high-res gallery photos. Look at how this one displays.
Here's an example. This would not only help with that layout problem, but also loading times as well. I realize high-res photos are great, but sometimes they're a bit too high-res when they're integrated into the website's layout. Some of the images are displaying well, such as
these, but you should go back through and make sure all your images are looking that way. I did not peruse the entire gallery, but there are a number that are too wide. In the end, though, a user shouldn't have to scroll right and left unless it's completely unavoidable... (they're running at 800x600 for instance)
-Nitpick: I know you like to display that your site is XHTML compliant and that it's just a beautifully standards-compliant site, but I'd advise against advertising this. What does it have to do with the client's site? That they were able to hire a proper web designer who can code to standards? Plus, not all your pages are valid.
Here's an example. (I clicked that link from one of the photo display pages). Also, shouldn't clicking the "CSS" link take me to the CSS validator and the XHTML link take me to the XHTML validator?
- Clicking beach houses and then clicking the picture above "View all pictures >>" throws an error. It links to an invalid IP address:
http://0.0.0.0/brittonstuff/brittonweb/gallerypopup.php?i=beachhomes/full/5; Condos does the same thing and so does Residential.
- The remodeling tab is broken, or the layout breaks for some reason, and it contains no sidebar image. That's a bit "shocking".
- Nitpick: I know that this is a nitpick, but I really like it when the website's main image links back to the root of the URL and not to its index page. ... ... I know, that's terrible...
On the whole, once you work out a few minor kinks, it is a great and wonderful site. Good job!
EDIT: Added a few points of clarification.