The tutorial applies to theme208 and above.
1) To add additional product images we should create a folder under /componenets/com_virtuemart/shop_image/product folder.
2) The folder name should be the same as SKU of the product we are adding additional images for.
3) To get product SKU you should login to your Joomla admin and navigate to Components -> Virtuemart -> List products -> Locate the product on the list -> Edit it and check SKU field.
4) Then you should navigate to /componenets/com_virtuemart/shop_image/product folder using any FTP client and create a folder. Use product SKU as folder name (case sensitive).
5) You should create 2 images for each additional image. First one is fullsize image and the second one is the thumbnail. Thumbnail file name should have small_ prefix.
E.g.: fullsize filename is 1.jpg. Thumbnail file name should be small_1.jpg.
6) Upload your additional images to /componenets/com_virtuemart/shop_image/product/your_product_SKU folder.
NOTE: do not forget to change your_product_SKU to the actual product SKU.
Feel free to check the detailed video tutorial below:VirtueMart. How to add multiple product images (starting from the template #theme208),