You can add somthing like this at the end of your json callback:
var marks = [{long: -75, lat: 43},{long: -78, lat: 41},{long: -70, lat: 53}];
svg.selectAll(".mark")
.data(marks)
.enter()
.append("image")
.attr('class','mark')
.attr('width', 20)
.attr('height', 20)
.attr("xlink:href",'https://cdn3.iconfinder.com/data/icons/softwaredemo/PNG/24x24/DrawingPin1_Blue.png')
.attr("transform", d => `translate(${projection([d.long,d.lat])}`);