How to tag a large number of images?

Question

Images

The bottom line - there are several tens of thousands of images on the site. Images are not unique - search engines look for copies.
The task is to give all images tags, so that when searching for the word "cat", images with cats, etc. are searched.
Nothing but writing your own service for issuing tasks to users with payment for these tasks came to mind.

Answer 1

Another option is to search for these pictures in Google and parse the results, sometimes it gives out the name or subject of the picture
, but I think there will be many errors, and the number of requests to Google