according to some investigation, geoportail stores tiles with the following url scheme :
the upper and lower case have impact.
http://visu-2d.geoportail.fr/geoweb/mapsUxGM2GGY.jpg
http://visu-2d.geoportail.fr/geoweb/maps8u6T23KrPmH.jpg

now I have to figure which java script is in charge of creating the url....
Best Regards,
JLM