To add an additional product image go to menu Products > View Products menu. Once clicked on this will display all products available in your store.
To edit a product click on the "pencil icon" on the top right hand side of your screen.
The product screen is split into tabbed sections: General, Specifics, Connect, Features, Variants, Images, Promotions, SEO. Click on the tab called "Images".
Now Click on the plus symbol Icon under the section "Additional Images". Once you have clicked on the plus icon a new section will appear to add your additional images. (you can repeat this step for multiple additional images)
Click on the "no image" image and you will see a pencil icon appear. Click on the pencil icon and a new pop-up box will appear. 
Move your mouse over to the third "(up arrow) icon" and click on it. You can then select a local image from your computer and upload it to your store. This will be your additional product image for this product. Make sure you save your product after adding the additional images.
You can also set a sort order number for your additional images.
