News to me! Photo Booth is included with the G5 iSight iMacs, too.
A review article from Oct 2005 - http://www.gadgetreview.com/2005/10/apples-photo-booth-turns-your-isight.html
Being a part of your OS X install, you could try an Archive & install of your OS X install. Could also be one of the bundled software, and you could try the restore DVD that shipped with your iMac. Insert that disk and install from the apps bundle that is on that disk.
http://guides.macrumors.com/Photo_BoothPhoto Booth was unveiled at an invite-only special media event in October, 2005, Apple released Photo Booth alongside the Rev. C iMac G5, the first one with a built-in iSight camera.