Hi, 

If you're using the framework, the HTML template boxMenu.hbs has an underscore ( _ ): 

<img src="{{_graphic.src}}" alt="{{_graphic.alt}}" />

This means that the HTML template won't display "graphic" as it's looking for "_graphic"

However, if you add an underscore to your JSON file in contentObjects.json, the images will appear

e.g. 

"_graphic": {
"alt": "alt text",
"src": "course/en/images/croissant.png"
},

There are no doubt other solutions to this, but this is one that worked for me.