Hi
You can do that directly from iPhoto. Just find the pictures you want in iPhoto and select them (click on each photo while holding down cmd) and drag it inside your USB.
It's that simple! You can drag photos from iPhoto everywhere! Inside pages, numbers, keynote, photshop, iMovie, Finder, etc...
You should never go inside the iPhoto library file, even if you're an experienced user, this could mess everything up. iPhoto is already buggy the way it is, no need to ake it worst.