As FADE is by default, how to add a NO FADE effect to an image ?

There are many different transitions, the no fade is called CUT, here's a list with the other ones: https://lunafromthemoon.github.io/RenJS/actions/showandhide/transitions/

Does this solve your problem? Also, theres a Defaults.js file, where you can set the default transitions for each kind of object.

Cheers!

Using CUT is what I need :) Thank you !