Alt text stands in for the image wherever the image cannot be seen: for a shopper using a screen reader, on a slow connection where pictures fail to load, and for a search engine indexing the page. In a webshop it is a field of its own beside each product image.
Useful alt text says what the picture shows, in a plain sentence: the product, its colour and material, and the view, such as a navy wool coat seen from the front on a model. It does not repeat the product title word for word, and it is not a list of keywords.
Each image of a gallery deserves a description of its own, because the front view, the back view and the detail of a cuff show different things. On marketplaces the field is often missing, and the title and attributes do this work; in a seller's own shop the alt text is the seller's to write.