You can add an image to a Product when creating the Product, or after the Product has been created.
Product images should be a min of 640px on the longest side. 900px is a good size for most templates.
You should ensure your images are compressed before upload to ensure you do not slow your page speed.
When you have chosen to add a product via the Products tab ( or eCommerce > Add Product), you can upload an image as part of the detail.
When you want to add or change a Image on the Product, navigate to the product via Products tab (or eCommerce > Edit Products)
Click the blue image icon to launch the image wizard to add or change the image.
You can add additional images to the product - either via the Photos tab, or by clicking the More Images button that appears next to the current image thumbnail, when editing a product.