Hi James,
Today, I had the time between two project to continue to work on this.
Thanks to your help and guidance, I made it.
As you can see with the selected solution, I use the first one as I’ve to “add” everything and have no “existing values”.
My format sightly differs:
$value = array(
array('before_image' => $before_image, 'after_image' => $after_image, 'acf_fc_layout' => 'before_after')
);
I’ll continue and hope everything will be ok!
Thanks again.