if you bought iPhoto from the Mac App Store, you can use iPhoto on any other Apple branded computer you own or control (for private purposes, not for commercial purposes). See the About panel for your Licence agreement. iPhoto > About iPhoto
You "are granted a limited, non-transferable, non- exclusive license:
to download, install, use and run for personal, non-commercial use, one (1) copy of the Apple Software directly on each Apple-branded computer running OS X (“Mac Computer”) that you own or control; and....."
If you bought the boxed retail version, the licence will differ. Then you can install iPhoto on one Desktop and one portable computer.