Did you know you can create instant slideshows in the Finder of OS X 10.4? I am not sure how far back this feature dates, but it does work in 10.4.3.
Simply select multiple images from any directory on your drive. Then right click (or control+click if your a still stuck with a single button mouse) on one of the selected images. Select Slideshow from the contextual menu. The screen instantly goes black and starts a slideshow of the selected images. Once the show has started, you can use the mouse to pause, cycle, or scale the images. You can even view an Index Sheet of the images making it easy to find specific slides instantly.
I could not find documentation detailing the image formats supported. Gif and JPG work fine while PDF and EPS do not. In any case, there is no need to open another application. The feature has been conveniently implemented at a Finder level.