I have updated my theme to version 2.6.6.
I have disabled all the plugins.
And the box to insert images (featured image) in the addition of slides, still does not appear.
Meanwhile I have tested to insert slides in a page using short code, but the images loose quality and they are blurred as you will see here: http://valledental.es/tarjeta-andalucia-junta-sesentaycinco/
I need a solution to create slides in my pages.
Many thanks in advance