You can add a product image using the designer.
Go to Design
From the merchant portal, choose Design from the navigation menu.
Open the Designer
Find the section for Published Design and press the Designer icon.
Go to the Products Panel
In the top menu, find the section for Pages and click Subscribe. 
You will see your new product on your page with a default image selected.
Swap in Your Image
Click the stock image, and a bar three icons will appear. Press the center Icon Showing Two Arrows.
This will open a gallery where you can either choose an existing image by hovering over it and clicking the green Checkmark, or upload a new one using the blue Upload New icon in the top left corner.
Save Your Changes
Before you leave the designer, press the Save button at the top of your screen.