Hey, not bad, the site is working pretty well.
I found navigation pretty easy. Although if it's a portfolio site I kinda wonder why you have that "main" page, and then also the "portfolio" page, since they have the same images on. I would probably just ditch the main page that it goes to ("home" - what is that actually for?) and just make the very first page be the Portfolio page so people get to all your art right away.
I'd also suggest getting rid of the flash viewer, it's much slower and less straightforward than just having the images inline in the page. What I'd do is, keep your main Portfolio page the same, but instead of each link (eg. "Vehicles") going to another
list of thumbnails, each of which opens a Flash viewer, you should just have all those large images inline in the page that the initial thumbnail goes to.
So I'd click on "Vehicles" and see all the full-resolution Caravan images in a list, so I can just scroll down like usual and don't have to wait for the Flash viewer to pop up. It means less clicks for the end-user, which is always good, and they get to see your full-resolution art in a much more streamlined (and more web-standards-compliant!) way.
Does that make sense?