Would you mind sharing your store's preview URL so we can assist on that? Cheers!
You probably have the 'Collage' setting enabled in the customiser, which will be the reason why there is the grey border around the images. But to make sure the images are consistent you need to pair the 'Grid' style with images of the same dimensions.
A problem here is the size and aspect ratio of the photos themselves. You need photos that are the same size and dimensions, they will then display the same and fit into the section correctly. It looks more visually appealing to have consistency within your images, which improves the users experience of your site.