Questions related to CSS

Yes, the lead image has w 100% width to fill its container – this is especially useful in mobile formats. You should upload a photo that is at least the width of your full width container (1140 pixels).

Or you would need to add some CSS to your theme’s stylesheet to reduce it down from 100%. Example:
.wplistings-single-listing .listing-image-wrap img {width: 70%; margin: 0 auto;}

*the margin is to center it in the container

The listings page defaults to three columns. Your theme is adding a 2.5% margin to all .entry div’s that overrides the one-third column margins.

Adding this should fix that:
.listing.entry {margin: 0;}