iPhoto - find all photos that look like this photo?
There's actually no way to do that in iPhotos/Photos.

But you should take a look at Google Photos which add this feature recently.

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).