quit iPhoto and drag the iPhoto library to an external hard drive that:
1 - is formatted Mac OS extended (journaled)
2 - is connected with a fast wired connection like FireWire, ThunderBolt or USB
3 - is not being used as a TimeMachine backup volume
and
4 - is always available prior to launching iPhoto
Hold down the option key while launching iPhoto and use the resulting select library window to select the new location on the EHD - fully test the library there and then you can trash the library on the internal drive - test one more time before emptying the trash
and verify that your backup software is backing up the EHD - some like TimeMachine do not by default and you have to remove the EHD form the excluded drive list in the options
LN