To add more images for a product first add the main image by clicking the button in Product Image box on product add/edit page.
After the main image is added the fields for additional images will appear.
To insert additional images click on Add Product Image button.
The image gallery will show up. Select one or multiple images to attach to the product.
After the insert button in insert media window is clicked the images will appear in product images box. That’s it! Save the product.
The default number of additional images for each product is 5. See how to set custom number of additional products.