Hello,

You can find following codes in /wp-content/themes/Travelo/functions.php file.

add_image_size( 'gallery-thumb', 270, 160, true );

And

add_image_size( 'slider-gallery', 900, 500, true );

You can change the image size enough large in it.

For example, 

add_image_size( 'slider-gallery', 1800, 1000, true );

After changed it, please regenerate thumbnails use the plugin https://wordpress.org/plugins/regenerate-thumbnails/

Best regards,

Good morning,

Tje quality of the destacate images is too poor, i upload a good quality image, and if you open the article the image looks good, but in the preview is too bad. how can i fix? 

add screenshot with the problem