Quote Originally Posted by dealbyethan.com View Post
I thought additional images will only be inserted to a product when we name the filenames the following way using underscore. Does hyphen work as well?
No and no. Additional images are any that share the same beginning as the base product name (i.e. less the extension) for a product. So in the case of a product with the image tee.jpg, absolutely any image that starts "tee" will be treated as an additional image for that product.

Hyphens, underscores and so on are just for good naming practise as they help to identify to you which images you intended as additional images.