It looks like Sharp integration by gargol · Pull Request #9837 was merged into Ghost 2.1.0. Now, there is an imageOptimization setting on the file core/server/config/defaults.json:
"imageOptimization": {
"resize": true
}
See discussion at [Investigation] Image Processing & Manipulation · Issue #4453, and the related Automatic thumbnailing/resizing of image uploads - Ideas - Ghost Forum.