Extra space for item photo

I am editing a simple online web-store and on the pages where we have more products the top line of items sit next to each other, then on the second row there is a wider product space even though we have only attached one image, so to the right there is a blank space. I only want to show one image. This stays the same regardless of which image or item it is – does anyone know why? I assume it is designed within the theme but I want to change it.

I don’t want to add a second image to fill the gap as if the customer re-organises the view of products for their preferences all of the images will rejiggle and the gap will still be there unless all items have 2 images, which I don’t want.

https://wordpress.org/plugins/galleria/