I had one for the US purchase of Alaska which I got from the images of Americans building log cabins in an icy landscape and another image showing an American signing a document. I assumed it would be either Washington or Alaska (Anchorage I guess), but wasn't sure which because it depends on if you weight the signing of the agreement over the building of US settlements. It could have been either given the images were of different locations.
Similarly I had picture of British dude creating telescopes and realised it was very likely Herschel. But I also knew Herschels early work was done in Bath, while his most famous telescope was built later in Slough. Again, it wasn't entirely clear which location it would have been referring to.
Maybe I'm just being stupid though. I think you could have argued that right answers in both cases were more likely to be Washington and Bath.
That said, I really really liked it and think you have something here. Personally I'd play this over Geoguessr any day and I'll show my GF it tomorrow because I think she'll also like the history aspect of it.
Also, might worth lowering the distance penalty if someone guesses the right country, but the wrong point? Events in large countries are more risky just because of their size. Eg, if an event happened in France but you click Germany you'll often get less of a distance penalty than correctly guessing an event happened in the US but clicking the wrong part of the US.