What's the whole set of tools available on review board?
The tools help you either annotate your screens or view your screens better. Just like your options in SnagIt, or Paint, only simpler.
Annotation tools include: comment, rectangle, pencil, arrow, erase.
Viewing tools include: pan, zoom-in, zoom-out, fit-to-width, hide/show. 
For zoom-in/out you can also use 'ctrl'+'mouse scroll'.