You have full control over how the slides play back. For example, you can pause, rewind or skip forward by using the interactive controller at the bottom of the slideshow screen. As in QuickTime's full-screen mode, this controller disappears after a few moments and can be brought back by jiggling your mouse. From left to right, the controls let you go back (left arrow), pause (pause/play indicators), go forward (right arrow), switch to an index sheet presentation (four rectangles), toggle between full-screen and actual size presentation (square with arrows pointing out or in), import images into iPhoto (photo with camera) and leave the slideshow (circle with embedded "x").
The index screen presentation allows you to skip between images. Just click the image you want to jump to. The display of your slides scale depending on the number of pictures in your slideshow. If you have just a few pictures, the images are large. If you have, say, 90-odd slides, the images shrink quite a bit. Moving your mouse to any slide causes the file name to appear for that image.