Flickr does not provide an official similar image search engine nor corresponding API. Nevertheless, CLEAR realizes it on a user-side.
CLEAR runs totally on a client side. It does not use a backend server at all.
CLEAR does not store any images nor build search indices, while traditional search systems require to build search indices.
CLEAR is good at images with an explicit object class, e.g., dog, cat, and bird, due to the backbone image feature extractor.
CLEAR is NOT good at portrait images due to the backbone image feature extractor.
Source code is available at
https://github.com/joisino/clear.
You can fork the repository and customize and run you own CLEAR.