Hi,

If you use a single post style with full width ( full to edge of window ) - you should use image with at least 1920px width.

If you use single post style with the featured image in a fullwidth container like before - you should use images with at least 1170px width.

Best Regards,

PenciDesign

Hi,

This issue caused by your featured image origin is small:

You can check origin image here or you can go to that page > click on featured image > you can see the origin image smaller with the image display at the top.

Because you're using that single post style - so, the featured image need to get the width fit with the container of that page.

So, please:

1/ Change the featured image to another image with the size bigger.

2/ Change the single post template to another style via Customize > Single Post Options > Single Post Template.

Best Regards,

PenciDesign

Hi,

I see your featured images has fine quality.

Example on this post - http://prntscr.com/njzdzk

We've loaded this image - It's quite fine for a website. Because we can't load full your images there - it will make your site very slowly to load...

Also, you should go to Customize > Standard & Classic Layout Options > and check to "Auto Render Post Excerpt" to make your first post shoter.

Best Regards,

PenciDesign