Add image property
Add a new property type, and from "type" dropdown, select "Image Upload" or "Image URL".
If you select upload, you'll be able to upload images to link with your node. Otherwise, you'll be able to paste URL of online image to link.
Use the property as image
After adding the image property, set it to become the "image property". That way, it will be used in the graph.
You can now upload images by dragging them directly into the table cell. Alternatively, you can double-click the cell, and drag images into the form that comes up.
See images in the graph
After creating the property, setting it as image property, and uploading a few images, the images will show in the graph.