Important images with products and for example categories automatically receive an alt text. The alt text of these images is the title of the product and / or category.
When there are multiple images, they are made unique by placing a number behind them. Because of these alt texts, images are better indexed by Google.
Add your own Alt-Title to an image:
You can also add an alt title yourself via Starting point > My Products > Manage Products, and then add an image to the product via the media tab, then you can enter a title / alt text in the image itself.