Hi,
If we have custom image like for slider on home page or any other location not part of product and default images used by prestashop then need to check alt text for the image, In case empty then we can use keywords in alt text of the image.
PrestaShop Tutorials Videos [How to do Tasks]
https://www.prestashop.com/forums/topic/907438-prestashop-tutorials-videos-how-to-do-tasks/