My store has hundreds of images with no alt text — bad for accessibility and bad for SEO — and writing them by hand is soul-crushing.
Design a prompt that, given a {{product_name}}, {{key_attributes}} (color, material, use), and an optional image description, writes concise, useful alt text: describes what's visually shown, includes the product and one or two distinguishing attributes, stays under ~125 characters, and avoids keyword-stuffing or starting with 'image of'. When {{need_caption}} is true it also produces a separate, slightly longer caption for the listing body.
A strong solution is consistent across a whole catalogue, never repeats the product name awkwardly, and treats a lifestyle shot (a person using the product) differently from a plain pack shot. Link the prompt you'd trust to run unattended over 500 images without producing spammy or duplicate text.