This tutorial covers the way you can add a featured image to a product in the menucard.
This same process will also add images to your gallery items, or put images at the top of your posts.

Step 1: Edit a post

Go to your wordpress admin (http://www.yourdomain.com/wp-admin) and go edit a post on the following page: “Pages”. For this tutorial, we’ll edit “Product” we’ve made in this tutorial.
You will see a screen like this screenshot:

Now click on “Set featured image” in the “Featured Image” block. You will get an overlay (picture underneath) where you can upload an image. When you’ve uploaded an image, scroll down in the overlay, and press on “Use as featured image” and press on the exit cross.

You’ll probably notice a picture now, in the “Featured Image” block. You’re set now. Press on the blue “Update” button to save the settings. If you browse to your menucard, you will notice a new thumbnail on the left of the product. You’ve set a featured image!