@Jellby and theducks
Thanks for your very informative corrections and proposals.
I was happy to obtain at first try a working solution in ADE. The fact is that I use images usually once. This time the same image has to be used 31 times and you are right to pinpoint that this asks for some adjustments.
I will now have also a more elegant solution. I will post a v2 ( v1 are just made for this).