I removed every image from WordPress Library and then re-attached each one to its own article, and that worked fine.
Thank you!
Update (2 hours later). I realized that there is an article in which Featured Image is shown, but two arrows are present as if it was a slideshow.
The pictures attached to the article are not shown at the top of the page.
https://tomsalix.altervista.org/2018-05-01/sede-clan/
