I think parent is more concerned with the way that after enough times seeing the same image and linking it to an answer, your brain does what your brain does best, which is find mental shortcuts. Gradually you come to associate the image with the answer, without regard to any kind of "why". This process happens rather quickly, and when you're faced with even only a slightly different image, you suddenly find it's very difficult to remember what the country is, despite blazing through it in your reviews.

Changing the image slightly helps force you to actually think about _why_ you know the image is depicting the country in question, rather than simply building automatic visual recall of a country name tied to a specific image.

You can of course achieve the same thing with just having a bunch of different "stylesheets" for your map or something, but that probably requires manual creation in order to not be a complete mess, and still probably isn't as good for building transferable knowledge. This approach on the other hand is much better at building transferable knowledge, but requires more frustration and a much longer time horizon to be able to usefully utilise the knowledge.