Technically there is no advantage over using php, html, xhtml or any other extension over the other. They all get indexed and they all get served up by the search engines. You can use html and it will be fine, .html is limited however to what it can do, but unless you need the functionalities that some of the other extensions give you, there is no need to change.
I too like the site and I agree with Dave that the images look pixelated. They need to be a higher resolution. I also like the layout of the site.
Weakness
Directory listings / indexes should not be possible. Some isp's have a button you can check somewhere in your control panel turning off directory indexing. A quick no frills way of doing this is to place a blank index.html in each folder that does not have an index.html. One url tested was
Index of /images .
I understand you did not want any comments on
SEO as you are performing okay in the rankings, but for what it is worth you may want to update your copywrite and also add a title to your text links to help with accessibility.