@Frimatek, thanks for chiming in! That CSS certainly does hide the images visibly from the page and gets the job done quickly.
@beatricemartini, one thing to keep in mind with the technique above is that the image is still physically present in your markup. Even though it is not visible, it is will be requested on every page load. If you want to take it one step further and ensure that it is not loading at all, you can overwrite any of X's views via a child theme. There is an in-depth article on customization best practices here that walks through all of this in great detail.
You'll definitely want to check out that article, but the basic premise would work as such (let's assume you're using Integrity): locate the view of what you want to overwrite and copy that file's contents to your child theme following the same path. In this case, you'd likely want to update the file at
/framework/views/integrity/content.php, which is the main output for the content of a standard post. You could copy that file to your child theme at the exact same location:
/framework/views/integrity/content.php, which can now be safely edited in your child theme. When you copy a template to your child theme, WordPress will always load that view first, along with your edits. Inside that file, you could do the following:
// Standard post output for Integrity.
<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php if ( ! is_single() ) : ?>
<?php x_featured_image(); ?>
<?php endif; ?>
<?php x_get_view( 'integrity', '_content', 'post-header' ); ?>
<?php x_get_view( 'global', '_content' ); ?>
<?php x_get_view( 'integrity', '_content', 'post-footer' ); ?>
if ( ! is_single() ) : condition statement wrapping the featured content. This will allow featured images to show up on your blog index but not single blog posts, which gives you the benefit of showing content where you want it, hiding it where you don't want it, and not loading any extra resources that you don't need.
Hopefully that helps to shed a little more light onto the usefulness of child themes and what you can do with them. Cheers!