WPShower Imbalance Theme: Spaces between thumbnails on front page

Hi,

The site I’m working on is:

https://site.otherhandmusic.com/`

I resized the images for the front page, but would like to narrow the space in between them so that there are an equal number of pixels between the images horizontally and vertically. The images are also pushed to the left and I would like them to be centred.

What bit of code do I edit, and what edits do I make so the images can be centred, with an equal amount of space between them?

Thanks in advance.