Fabrice,
I downloaded the basemap file and copied it to MyMaps. It appeared in the database of QLandkarte M, and on the screem at zoomlevel 23.
With ozi2geotiff I converted an ozifile to geotiff. I put it in the same map as the basemap file. It didn't show up in the database.
I compared the two files with gdalinfo. The structure of the files were very similar.
It is, however, a large file. The ozifile was about 3 Mb, the geotiff 18 Mb.

Thanks for helping me out !