I thought that too but what if you have numerous images promoting a product? Can't have:
- product name (an a unique identifier) for image 1
- product name (an a unique identifier) for image 2
- product name (an a unique identifier) for image 3
Wouldn't it look spammy to have numerous images on a product page with the same keyword as the title?
In my view, images on a product page should enhance the product, they should demonstrate the product in action to further incentivise the customer to go "aha..this is a great product."
For example a product like a shovel would have images showing the shovel in usage.
This is why I thought the description would work as it describes the image, is unique and features the product name. But yeah, it's the length that concerns me.