How to remove alt tag of additional image in product info page

HI

My site have been submitted to google for 7 months, till now, only index page was listed....

I am still finding where the problem is. The poor listing may be because I have 8 additional photos for each product in the product info page, and each image automaticlly generate the same alt tag, I suspect that search engine thinking that I am spamming it.

May I know how I can edit, or at least delete the alt tag for the additional image?any other better method to solve this potential problem?

Thank you