How to add space between rows of WP gallery?

I’m using the WP built-in gallery to set up a very basic shopping cart, and the rows need some blank space in between them or people might get confused about which “add to cart” button belongs with which painting.

WP’s gallery doesn’t come with many options, but I’d like to use it anyway. Do I need to edit the photos so that the jpg file actually includes left, top, and right (but not bottom) margins as an integral part of the image, or is there an easier way to style the gallery arrangement?

I’m grateful for any help. My page is here.