Thanks ohn Huebner for your answers.
I finally find a solution!!!
The code that worked is this:
$imgcategory = get_field( 'imagen_de_categoria', 'category_'. $category->term_id ) ;
echo '<div class="menu_section_parallax" style="background-image: url( ' .$imgcategory. ' )"></div>';
I´m not sure why it needs the ‘category_’ parameter: if you know it will be very helpfull for the future (my learning future) if you can explain it to me.
If you can´t, thank you anyway!!