All, is there a way in iPhoto to find all the photos that look like a given photo? I suppose I could use Face, but it's actually not for a face, so that's a bit strange. For example, I'd like to tag my pet's photos with the pet's name. I also would like to find all the photos I took of plated food, etc.
There's actually no way to do that in iPhotos/Photos.
As far as I know iPhotos/Photos can only identify human faces, geotagged photos, panoramic photos (Photos only), in addition to various metadata (such as date, description…).
Nevertheless you can add tag manually (at least in Photos.app) by opening information related to the photo (cmd+i).