1. Go to the 'catalog' menu and choose 'categories/products'.
2. Click on the category that contains the product that you want to add images to.
3. Next to the product name on the right-hand side of the page, click the 'i' symbol.
4. Choose the product you want to add images to and click 'new file'.
5. Under 'default image file' choose the image you want to add.
6. Click 'save'.