There's a common misconception that ALT tags should be used for SEO. They should be used for Accessibility, first and foremost. An ALT tag will be read by screen readers to help people better understand what's being presented to them. If you're thinking "blind people don't shop" you're wrong, very wrong.
What is an ALT attribute?
The ALT Attribute, commonly referred to as the ALT Tag, is text that is displayed to screen readers, or on the page, if an image fails to load. It's purpose is to provide the end user with an understanding of what is in the image that they are unable to see.
Yet merchants are mainly using the ALT attribute for SEO and is causing more harm than anything else. What's worse are the number of SEO websites telling people that's what these are for, when they are not.
Not only do lots of websites spread this misinformation, many apps also propagate this myth by doing things like generating ALT tags along the lines of "Product Name | Business Name" on all the images.
In the example below, which text would help you understand what is in the image that didn't load?
With the basic example above, you can start to see how this will create poor experiences for users using screen readers or if an image doesn't load.
Action Figure, Action Figure, Action Figure
Imagine going to a website to look for a product, you get to the Product page and all the images are exactly the same thing, but none of them actually show you the product. Now you know how people with screen readers feel when your ALT tags are all the same.
Each product image should have unique alt text to describe what somebody should be expecting to see here. You'll want to be detailed, but concise. Informative, but not excessive. You still want to fit it within 100 characters or less.
But what about SEO?
What about it? You can still include keywords and phrases as part of your ALT attributes, but Google isn't going to rank you better for this, it just provides additional context to the rest of the page, but more importantly, provides the description of the image!
You can also use the Title Attribute if you need to provide additional context outside of the ALT text.
Your ALT attributes should paint a word picture. Close your eyes and picture "man on horse". Not very easy to picture, too many unknowns.
Now try closing your eyes and picturing "man riding horse across field into a sunset". I still have the same keywords, and you can better visualize what is in this photo.
As I state often, focus on what is best for your customer. If your customer doesn't get the experience they are looking for, they'll probably just go somewhere else.