What can I add an ALT tag to?
An ALT tag can only belong to an image. You can't add an ALT tag to a page. For example, an image of a dog on your homepage can have an ALT tag, but the homepage itself doesn't have an ALT tag. The homepage contains many images, with each image having its own ALT tag.
All images should have an alt tag, regardless of where you place them on your shop. Not only does it provide search engines with information about the image but this could also help you and your shop to show up for a particular search query.