Jan Jan Jan... you're so helpfull that I canno't shout about you
I first empty the "component" file, and it doesn't change anything.
Changing the tamplate to the "standard" one made the image goes in. The strange thing was that it cames in with the com_phocagallery\assets\phocagalleryieall.css completly empty...
so I put the "right" phoca css at its right place and I modified the "module" iecss in mod_phocagallery_random_image\assets
and... it works!
it's not perfectly verticaly centered but I will check for that later.
Thanks again for your time!