I'd need to build a gray image of Switzerland based on elevation.
For instance, an image of 2000 x 500 pixels,
and every pixels represents the elevation, with value 0 for -300 meters or less, and value 256 for 3000 meters or more.
Can someone suggest me how to achieve this from the openstreetmap free data?
OSM data does not contain elevation contour data ... but have a look at http://wiki.openstreetmap.org/wiki/SRTM .. maybe this helps?