After a long debugging phase of our external RAM where we encountered many issues from the simple bad solder to timing issues we finally manage to deal with it.
For the moment we still have only one RAM which has five unavailable pins. So we can store only a eighth of on image but it allows us already to display symmetric pictures like this RoseAce!
You may notice that as we expected the center of the picture is much brighter. So we will have to change the parameters of our LEDs in order to be homogeneous.