Featured images with wrong width

Hello,

I’d like some help for my personal website [ Redundant like redacted ], where I use the free version of ElegantWriting theme by SuperbThemes.

The problem concerns featured images: within every single blog post, despite the width of the image displayed is 900px, a thumbnail is used that has the width of 768px, so the image is blurred. If I open the post image in a new tab, I see that the dimensions are 768x***. The same thing doesn’t happen in the loop of the blog, where the original image is taken and displayed. I would like in either case the original image was used or at least a thumbnail equal or greater than 900px.

In the functions.php file this code is contained:

function wpse_setup_theme() {
	add_theme_support( 'post-thumbnails' );
	add_image_size( 'elegantwriting-full-thumb', 900, 0, true );
}

I hope I was clear enough and you can help.

Thank you

Francesco

The page I need help with: [log in to see the link]