Facebook's photo viewer just got two overdue improvements. First of all, Facebook now supports high-resolution photos. When you click on an image in the photo viewer, it will display images at the highest resolution possible, which could be up to four times larger than the old size depending on the size of your display (the maximum size is 2048 x 2048). While you've been able to upload hi-res images for some time it's only now that you're able to actually view them in their massive glory.
The photo viewer is also getting fullscreen view, which you activate by clicking on a familiar-looking expand button in the top right corner.