Back in 2014 or 2015, I added a feature to my editor that allowed creating a map from a BMP image. It wasn't the first time that had been done, but it allowed arbitrary map sizes, and allowed more colors than a 1:1 ratio to terrains, and was thus more flexible than previous options.
Arbitrary map sizes... limited only by Civ3's well-known tile limit, of... 362x362 tiles? Not so! There is a Civ3 limit, but the limit is 65,535 tiles, or 2^16 - 1, and 362 is simply the largest number you can fit into a square Civ3 map (or square in an isometric sense, at least), and so that was the limit that the Firaxis editor made, and to my knowledge there hasn't been a map with more than 362 tiles in a single dimension to date.