How do you re-install iPhoto on OS 10.8

I am getting an error message when I try to launch iPhoto. It says I can't launch iPhoto because it may be damaged or incomplete. I recently installed iDVD from a later version of iLife because it did not come with my retina display macbook pro. I am not sure if I messed up iphoto in that install... but i have been reading all of the threads on what to do.... and everyone says to reinstall iphoto.

There were no installation disks with my computer because it did to come with an optical drive.... how do I reinstall?