Last thought... Don't you have a disc (that would have come with your iMac when you bought it) that's got iPhoto 7.1.5 on it?
Seems like you should be able to revert using it.
Good point about the "free" Yosemite upgrade not necessarily being all that free.
Edit: Caveat... iPhoto 8 may have updated your photo library and made it incompatible with iPhoto 7 (similar to what happens with iTunes); I don't know if there's a workaround for that other than re-importing all of your photos.
Last edited by artie505; 09/25/15 10:49 PM.
