These programs can analyze your page load time and the assets you use on your page. (It helps to reduce the load time of the page by analyzing your page and offering alternative solutions.)
Graphic rollovers mean for example when you use 2 picture for your menu button, one of them is the onmouseover phase and one for the “off” phase. Generally there is a JavaScript what changes this pictures. In this case the browser has to download the proper states of the button at every page load.
Pictures take more time to load than text menus, that’s why if you want to use graphical menus you could use CSS (Cascading Style Sheet) to make your menu. The browser chaches the CSS (in which you define your buttons like backgrounds) so the browser doesn’t have to download the CSS all the time.
Programming websites with CSS has a learning curve and at this time it could be frustrating because every browser renders the page by their own algorithm and what looks good in Internet Explorer may be crashed in Firefox and vice versa because of the lack of the standards. (Mostly IE implementation of the box model).
Hope this helps. 