Starting from ZoomEffect v3.1 (the latest ZoomEffect version that was released a week ago for VM3.0.12)
To match the PICTURES <-> CUSTOM_FIELDS, we started using the "Image Alt-Text" field, instead of "Displayed image subtitle" field. It is because with VM3.0.12, the subtitle field for images becomes important & useful. We thought people might want to use subtitle field for some other purposes. But the alt-text on the other hand is a very useless and won't harm to website if we use that field for matching (FYI: According to Google's official Tips&Tricks articles, alt="" attribute for pictures have now no effect on SEO. it was in the past, now things have changed. Google ignores the alt text of pictures, instead looks at the general text content that surrounds that picture to figure out what that picture is about)
If you don't have too many products, then simply copy/paste the CUSTOM_FIELD id numbers from "Displayed image subtitle" to "Image Alt-Text" field for pictures.
If you have too many products (let's say 100) and if it would takes hours from you to do this field change. Then don't touch the fields and try this patch:
Open the new default_images.php from ROOT//templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/default_images.php
and look for this expression (you will find it in 2 different lines):