This can be done via the Business Dashboard using the Menu Editor under Online Ordering.
Follow the steps given below:
1. Select Menu Editor under Online Ordering on the dashboard navigation list.
2. Find the Item you want to add an image for by picking them from their relevant Category / Sub Category and select the same.
3. On selecting the Item, you will see the Item Details tab for the same, scroll down to Item Image:
4. Click on Browse Images and you can add the Image.
5. Once you've made the edits click on Save.