yeeeeeeeeeeeeeeeeees!
thank you!
the "choose this image as album cover " had just done the trick.
So evident I didn't saw it

And the shuffle mode from the reset thumbnails page is a way back.
This way one can switch between fixed and random images with the same function.
Each step I make into that soft proves me its totally clever conception - the code documentation is also a masterpiece.
As a webdesigner popping out off a loads of bloated web apps, coding with zenphoto is a holyday trip.
Thanks again Mr billard, Mr acrylian, and the others regular contributors.