What is the difference?

Different recipe is different

"Coca-Cola Zero Sugar looks and tastes more like Coca-Cola Classic, while Diet Coke has a different blend of flavours which gives it a lighter taste."

What's the difference between using that reactionary image or the Galifianakis one?

The DIFFERENCE is when they say zero something they are telling you it is 0.9 or less of that. Where if it say NO something it means none.

the sweetener. Haven't we gone over this?

There were mainly two different but very similar versions created because studies showed that men refused to drink diet coke because it said "diet" which they found is mainly for women. True story.