I use GIMP. There is a
Mac version. You need to first reduce the size to around 1000 to 1200 pixels wide otherwise you have to reduce the quality much too much.
What I really would like is a plugin that runs on Win64, Linux and Mac where you could say "I want a jpg/png/whatever no bigger than 200KB. Reduce the size to preserve 90% of the quality." Otherwise it's trial and error guessing the size and quality.