From there you have a couple options to bring up the slideshow feature.
You can press spacebar for quick look and it will bring up this:
You can now use the arrows to scroll through the images or click the thumbnails to bring up thumbnail previews.
Now at this point you can press the fullscreen arrows on the right side. Doing this is the same as selecting all the images and going to finder menu and while holding down option select File > Slideshow. So if you want to bypass the quick look then you can just go right to the finder menu instead.
After clicking on full screen you will your first image in full screen mode and this toolbar at the bottom.
You can scroll left or right through images or press play for slideshow. You can click the thumbnail view or click import to iPhoto.
This is a very nifty feature especially if you have thousands of images on your hard drive and need to locate one.