I browsed to your web page and checked its source code. If you browse to the same web page in Internet Explorer, and you hover over the Home/About... menu items, it will show an Error icon at the bottom of the browser. If you double-click the icon, then, it will show the same error message. This error message may not be shown in Firefox or Chrome.
To fix the issue, I think you will have to define the following functions - MM_swapImage(), MM_findObj(), MM_preloadImages() and MM_swapImgRestore().
Hope this helps. Cheers.