What is the best way to allow for multiple image uploads?

What is the best way to allow for multiple image uploads?

For an e-commerce site where a product can has one primary photo plus multiple secondary photos, what is the best way to enable uploading multiple images?

1. If you upload multiple images one at a time just like a single upload app, what naming conventions should you follow in order to store the images linked to a particular product? E.g. product a1b has images a1b-1, a1b-2 etc. Is using an end number useful? Could this number go into double or triple digits if necessary?
2. How can you enable users to upload multiple images in one go? E.g.
3. How can you differentiate between the primary and secondary images? How do you update the primary image?

Question 1 is of the highest priority for me right now but I will also want to consider the other questions in due course.