This is what was produced by the GeoServer instance.
But it did not work (or has insane results in height) and it seems the image from readymap servers is not a jpeg (even though it is named when requested) and is a tiff file, right? is jpeg supported? What is the required encoding/setting?
Single-channel GeoTIFF (16-bit int or 32-bit float) is the only supported format for elevation data. What you are seeing in a visual rendering of the elevation dataset (heights mapped to a color range) but osgEarth requires actual raw elevation samples.