After you have clicked on Edit Image at the bottom of your post, you will land right inside the designer.
Follow these steps to add a Shape to your layout:
1. Click on Elements and then on Shapes.
2. Choose the one you would like to add to your layout
3. You can change the color of the shape, opacity, and even layer it under the product from the right panel
4. Click Save after you have made all your edits.