A big thanks to Quizzer6794 for teaching me how to avoid horizontal gaps between pieces, eg see his US State Picture Puzzle Game #3... does anyone know how to remove vertical lines too?!
It took some time but not that long actually! KoljiVriVoda told me how to use a single image file that can be divided, so I have an Excel file in which I combined citypopulation list with simplemaps.com coordinates (I had to fix some city name differences by hand). Then estimating latitude/longitude of the corners of the image file, Excel worked out in which 'piece' each city is in. With some other formulae type-ins get combined when more than one answer is possible, so now all I need to do is copy the quiz, change the 'formula' on Excel (eg I want the 'latitude' block of the city to be at most 1 away from the piece to be considered), import the answers and then 'map grid to answers'. But having just read PSTSCCH's I realized how silly I was in not thinking that I could have used a simpler approach for this one!
I respect your enthusiasm. However, I think that if you are going by latitude, it will look more beautiful if you use one long piece for each latitude cohort instead of a set of smaller pieces. Also, doing so means that you don't have to worry about vertical lines.
whoops that was silly of me, got too used to 'copy and paste and map grid to answers' that I didn't think of that... thanks for your suggestion, you're absolutely right!