More or less the bottom bracket is simply what holds the cranks on. For most modern bikes you can get a a cartrage style with the bearings built into the unit like this: http://www.nashbar.com/bikes/Product...0052_194724_-1___

Or you can upgrade a bit to ones with outboard bearings that have the cranks attached to the chainring and it slides through like this: http://www.nashbar.com/bikes/Product...0052_502313_-1

There are other styles but not for the bike you pictured.