Thanks Andy for your answer. I tried this code but nothing happens on the mobile version. I do not understand where the problem is, this is suposed to be responsive, While I was doing several trials, I changed the settings ot the background picture from parallax to scroll, and this way I can change the part of the picture that is shown on the mobile version, but still, it takes only part of the picture, it is not showing the full picture in the mobile display.
I wonder if there is another way to ‘tell’ the them to take a less big picture for the mobile version instead of taking a part of the full 1310 px picture. Any ideas, please?