There are a number of reasons you might choose to change the location of your iPhoto library. Maybe you want to store pictures on a firewire drive, or maybe you want to store them on an iSCSI LUN, which I described how to work with recently. Either way, there are two ways I typically see people go about changing the location that iPhoto uses to store data. The first is to actually create a symbolic link from ~/Pictures/iPhoto Library to the directory you would like to use. The second, which is a better option is to go ahead and edit the location that the iPhoto preferences set as the path to the library (you would replace /Volumes/VolumeName/Path with the actual path to your storage):
defaults write com.apple.iPhoto RootDirectory /Volumes/VolumeName/Path
Once you have defined a new location, if you want to revert back to storing photos in your home folder you can then use the following command
defaults write com.apple.iPhoto RootDirectory ~/Pictures/iPhoto Library
or
defaults delete com.apple.iPhoto RootDirectory
