Hello again,
I've found the solution to my problem quickly and easily, so I'd like to share it with you.
I've noticed that the new installation of PS 1.5.6.2 according to the steps described by "PrestaShop Legend" did not have on the backoffice images in the form of "home_default", "large_default" etc but instead I had "home", "large" etc.
So I created new image types with the correct names and afterwards I asked PS to regenerate thumbnails and everything worked fine!
Kind regards,
Marianna