This you are asking us 100% custom modifications
(because you want to change how the things work/look) and unfortunately we don't provide free support or step-by-step instructions for custom coding modifications.
You may purchase premium support:
www.flexiblewebdesign.com/premium-support
and let us take care this modification for you directly on your website though.
TIP: Try editing these PHP file:
for Product Details Page: /templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/default.php
for Category Page: /templates/YOUR_TEMPLATE/html/com_virtuemart/category/default.php
and edit the PHP lines to change the image ALT tag from filename to productName. But if you have limited experience on PHP coding, then i recommend you not to touch anything
Regards.
Support