Re: Image width
Fixed-width themes are evil (unless content is strictly controlled). I don't know a way to limit image width. It's just my opinion, but I think it'd be better to make the theme flexible.
One possible workaround would be to give the wikitext or other center column div a width and an overflow:scroll property (in CSS), so anything wider than the width (like a wide image) would put scrollbars on the div. This would prevent the theme from breaking, but make the center column behave like an iframe, which might confuse users since they'd see the cut-off image but maybe not the scrollbar down at the bottom of the page, if the page is taller than the browser window. I guess you could test it and see how well it works.
-- Gary