Oh wow! Thanks @anthonysalamin! The explanation was very clear and comprehensive. 
I’m sure your code works perfectly and I’m doing something wrong because still doesn’t work on my project 
I’ve tried and replicated with my 
options but it doesn’t show anything 
not even the static image.
What I’ve done also was a trial with an empty page with only the HTLM and the code (in the body), checked many times the options and they seem ok! I can’t understand.
Maybe I can try with another system. Do you think it could work as well with an external URL for the displacement map and the picture?
Sometimes I use https://imgbb.com/ to save and then copy-paste the image’s address.
Anyway thank you so much 