Images Not Displaying on Masunirin Theme

This is my website, and I’m currently using the custom field to display images, but on the features it says you don’t have to use the custom fields to include a post image, however whenever I post using custom fields or with images inside the post, no thumbnail shows up on my main page. This is what I want my page to look like, and this is a link to the installation instructions. Can anyone offer help?

I’ve found out that it’s a theme related problem, because when I switch to a different theme that supports post images on the front page, the images work fine.