If I understand your post correctly you are trying to figure out the best way to optimize your alt tags for different versions of the same image. Alt tags should really focus on the keywords you are targeting on a page. I can give you an example from my site:
Product - Generac 6244 20kW Guardian Series Home Standby Generator with Aluminum Enclosure and 200a Transfer Switch (That is a mouthful)
Keyword - Generac 6244
URL - apelectric.com/generac-6244-20kw-guardian
Title Tag - Generac 6244 | 20kW Guardian Generator | Generac Generators
Image Alt Tag - Generac 6244 20kW Guardian Series Generator
Meta Description - The Generac 6244 has been our customers go to 20kW home standby generator. This unit is in stock and ready to ship same or next day for FREE. Call us today for more information, 123-456-7891
The image alt tag should reinforce your targeted keyword and include a shorter description than your description. Focusing on putting the most important part of the product first, then a secondary description and perhaps then a size. You don't want to have several of the same alt tags on one product, but sometimes this is unavoidable. I would say break the more specific description up between the numerous pictures. In my example I would do something like this:
1 - Generac 6244 20kW Generator
2 - Generac 6244 Guardian Series
3- Generac 6244 Home Standby Generator.
The alt tags are a good place to reinforce your keyterms and let the engines know the whole page is really centered around that specific product. It is also how Google Images display certain pictures, they use the alt tags.