This demo shows how you can use graph setting customMarker to set an image for graph's legend marker.
Please note, that legend's markerType should be set to "none" in order to avoid an awkward X to be displayed over the marker when graph is toggled off.
As an advanced topic, below is a version of the same demo, with legend markers being replaced with alternative image when they are toggled off:
See the Pen Changing legend custom markers on graph toggle by amCharts (@amcharts) on CodePen.light