> From the outside, it might seem as if he is enslaved by his pet.

I don't see the issue, this is the explicit contract of having a cat for a pet.

> explicit contract

I think many first-time cat slaves would disagree

They'll learn.

It took me all of maybe 20 minutes

Brute force pattern matching would be cataloguing all possible matching pixel patterns. The whole point of the ML approach is that with a comparatively few images (yes even 20k is comparatively few) the computer will correctly match pixel-sets it has never encountered before.

If your comparison function returns a float instead of a binary value, then you don't need to match against every possible image. Instead you just return how much each image is "similar" to the given image. A simple approach is to use cross correlation for the similarity.