You can get to this part of the system by clicking on the Products link in the blue Admin Toolbar and then clicking New Product > Images Tab. You can also quickly open any existing product and get to this tab by going to Products > Products, searching for your item, and then clicking on its title and then the Images Tab.
You can add as many images to your product as you like. The leftmost image area will be the Primary Image, that will be used as the default image for the product everywhere it is displayed. Any other images you enter will be accessible on the detail page for the product on the website, using thumbnails under the main image.
If you need to change an image, just click to select a new one from the server or upload a new one from your computer, and it will replace the old one. If you no longer need an image to display, you can click to Delete it, and when you save the product, that image will no longer be visible.
As with all product images, you will want to use either a Jpeg or a PNG file, and you will want it to be a minimum of 800 pixels in its largest dimension. For best results, anywhere from 1,000 to 1,200 pixels is a great size for your product images.