Changes not showing up when testing your pages in Preview or Preview in Browser is usually fixed by clicking on the Refresh button. This is because browsers are designed to keep previously viewed versions of a page in their memory (cache) to speed up the display of regularly visited pages. Preview uses "Webkit" - a core part of Safari - which is effectively a cut-down version of the full application, and also behaves in the same way.
Clicking on the Refresh button forces the browser (or Preview) to display the latest version. In extreme cases, you may need to clear the browser's cache - in Safari, go to Safari>Empty Cache; in Firefox, go to the Firefox Preferences, click on the Network tab and click on the "Clear Now" button.
The problem with graphics not updating in Freeway's preview mode when using Safari 4 beta has been fixed with the release version of Safari 4.